Now that the rumors have turned out to be true, what is this going to mean for Linux -- if anything?
Well, let's look at the facts that we have so far. Linux has taken off in large part because it runs on commodity hardware (Intel and Intel-compatible), and provides a Unix-like OS that's great for a lot of tasks -- and much cheaper than its proprietary Unix cousins. Linux has a solid presence in the server market, and is developing a presence in the desktop market.
Apple, on the other hand, is working on developing a presence in the server market and the desktop market. While it's widely held that Apple has a wonderful desktop OS, it only runs on (pricey) PowerPC hardware from Apple and doesn't run all the apps that people are used to from Windows.
Apple's move to Intel isn't going to change much. Firstly, Apple seems poised to continue its exclusionary stance, and will require users who want to run Mac OS X to buy the whole kit and kaboodle from Apple. The ZDNet piece quotes Apple Senior Vice President Phil Schiller as saying that "We will not allow running Mac OS X on anything other than an Apple Mac." This means that Apple computers will still continue to carry a price premium that many users are unwilling to pay unless they're already convinced they want to run Mac OS X. Even Apple's low-end Mac Mini is still high-priced compared to similarly-equipped Dell computer. (Apple users can now abandon the "but PowerPC is so much better than Intel!" since even Apple is giving up that line of reasoning.)
On the other hand, users can try out Linux today on the computer they're using to run Windows. No serious financial risk involved, download a few ISOs or buy a boxed set and they're off and running. If it doesn't work out, they can go back to Linux secure in the knowlege that they won't have wasted much more than a few dollars and the time to test out Linux.
Also, the transition will be taking place slowly. Even if the move to Intel hardware makes Apple price-competitive with Dell and other PC vendors, it will be some time before it happens. A lot can happen in two years, and Linux will only continue to improve on the desktop while Apple works on their "Leopard." (Prediction: When the next OS X is released, we'll be inundated by headlines built around the "Leopard changes spots" theme.)
As an unwanted side effect for Apple, the Intel announcement may have a negative effect on sales in the meantime, since users may not want to buy a Apple computer based on the "old" architecture. Apple can assure users all they want about "universal binaries" -- whether it's logical or not, many users won't want to buy a computer that's already seen as discontinued. Pre-announcing products always means that a certain segment of your market will hold off until the new goodies hit the shelves. Yes, two years is obsolete by some people's standards -- I tend to buy a new computer every eight months, on average -- but that's hardly the norm. (Just in case you were wondering, my last purchase was a 20" iMac.)
From this vantage point, it looks like the net effect for Apple is going to be break-even at best. I don't think Apple's move spells doom for Linux on the desktop, or poses a serious threat to Linux in the server market, and it certainly doesn't mean that Apple is poised to defeat Microsoft on its home turf. If Apple planned to allow users to run Mac OS X on any x86 machine, that might be different, but as long as users are obliged to buy an entire computer just to run Mac OS X, it's not going to take over the world by storm. You can argue till the cows come home whether it's a good strategy for Apple to tie the OS to their hardware, but the fact is that it's much harder to convince someone to invest $500 and up on a new computer to run an OS vs. $129 to install it on their existing machine.
By the way, Apple may be abandoning the PowerPC processor, but Yellow Dog Linux (YDL), one of the premier distros for PowerPC machines, plans to stick it out with PowerPC processors. It should be interesting to see how YDL fares once Apple stops selling PowerPC-based machines.