This is going to be the Mac Pro article, isn't it?
So you've got your iMac, which just saw a dramatic redesign, the Mac mini (which also saw a redesign last year when the optical drive got removed) and the Mac Pro, which I'll get to later.
The iMac and the Mac mini
I'm confident in saying that neither product is going to leave 2013 looking any different than it does now - the iMac design goes through 2 year cycles in design and the Mac mini iterates even less frequently.
Both machines use desktop-class Intel chips, so they'll get the new Haswells when they arrive. Both will also get the new integrated graphics (I think its called the 4790?). The price for these components will set the price for the machine - if they're more expensive then Ivy, they'll go up, but since they're going to be the first machines to ship with Haswell architecture I don't anticipate they'll have to pay a significant premium, so we'll see a flat price structure across the board, with no major increases or decreases.
What I do think will happen is that Fusion drives will become standard on all Apple desktops.
Anyone who's used an Apple laptop with an SSD knows that going back to a spinning disk can be painful. The speed of every task just gets faster, from opening applications to running processes. The transition to SSD was the last step in achieving truly instantaneous computing, and Apple would love to get an SSD in every computer they sell.
While the storage requirements in laptops permit the jump to SSD-only storage (evidenced by the adoption of the MBA, which at its highest level comes to 512GB of storage, or half of what the iMac ships with standard today), desktops have a reputation for being media hubs and archives for all sorts of content, creatively, professionally or personally. That's why the Fusion drive architecture hasn't made its way to laptops - having 1TB of storage has never been necessary for enough customers to warrant the speed compromise. Fusion on desktops gives the average user the speed in loading apps and starting up that an SSD provides, without forcing them to pay the exorbitant amount that 1TB of flash memory still costs. When that price goes down, maybe we'll see more adoption of flash in the desktop, but not yet. Maybe next year, in the iMacs new redesign (think of how much slimmer the iMac could get with no spinning disk. It would get rid of that hump in the back!).
So, Fusion drives will make every iMac and mini that comes off the lot feel much, much faster than its predecessor, and factor in Haswell and new graphics and that'll make it a substantial upgrade.
The Mac Pro
This is the email Tim Cook sent to a pro customer after the event last year where they introduced the rMBP (emphasis mine):
Franz, Thanks for your email. Our Pro customers like you are really important to us. Although we didn’t have a chance to talk about a new Mac Pro at today’s event, don’t worry as we’re working on something really great for later next year. We also updated the current model today. We’ve been continuing to update Final Cut Pro X with revolutionary pro features like industry leading multi-cam support and we just updated Aperture with incredible new image adjustment features. We also announced a MacBook Pro with a Retina Display that is a great solution for many pros. Tim
That's what makes people think the Mac Pro is going to see a major update in 2013, but theres obviously many ways to read that line, as he didn't explicitly say that it would be a new Mac Pro, just something "really great" for pro customers. What makes me think that we will in fact get a new Mac Pro is, once again, Haswell.
The Mac Pro line uses the line of Intel processors above the iMac and the Mac mini, called the E-series. The E's promise server-grade power in a desktop architechture, pushing an almost unnecessary amount of power for traditional desktop tasks, but designed to make more processor intensive work much easier. This is for modeling, video editing, intensive graphics work and so on.
Currently, the Mac Pros are running on the Nehalem E-series chip, which was the generation before Sandy Bridge, making the chips inside the current generation Mac Pro one generation old (there were no Ivy Bridge E-series chips). If they make a new Mac Pro, it is with complete certainty that I can say the next generation will have Haswell E-series architecture in it, making it undoubtedly the fastest and most powerful Mac they sell.
I can also say with certainty that, if they make a new Mac Pro, it will boot from a Fusion drive, standard. The Haswell chips will necessitate new motherboards and internal architecture, so it will have Thunderbolt (at least 2) and USB 3.0 (at least 4). It will have NVIDIA's latest desktop graphics card in it, so if it needs to power an external Retina display, it could.
I'm also going to come out and say that it's not going to ship with an optical drive.
Not having that is going to enable them to slim the profile down a bit. It'll be smaller, but not miniature - it's still going to be underneath a lot of desks, so it doesn't need to shrink, but it will slim down. That product will make people who were waiting for a new Mac Pro happy. It's modern, fast and just what they expected.
All that is predicated, of course, on whether or not they do one at all, which is a large enough question in and of itself. Millions of pixels have already been rendered on this issue, my favorite being from John Siricusa on his blog (which he's updating more frequently now that Hypercritical has ended, thank god). Like most educated Apple speculation, there are valid and realistic reasons for them to go either way.
Personally, I think they will, and we'll actually see them announce the product I described. I'd compare it to the iPod shuffle, or even the whole of the iPod series under the iPod touch. They'll never really sell enough units to justify their attention, especially when compared to the size of the rest of Apple's markets, but they serve two purposes: first, they instantiate their presence in a market that they have always been present in (it "keeps their foot in the door"), and second, because it provides a design testing ground for future products without sacrificing the valuable space of their more popular products. The least expensive and most expensive products serve as an engineer's and designer's playground, and that's too valuable now to give up for the sake of cleanliness.