I read some of the Apple w/ Intel rumors this past weekend but pretty quickly I stopped because there was so much speculation and today's WWDC keynote was supposed to definitively answer the question. So Steve just finished the keynote and it's true, Apple is going to move away from running on PowerPC chips to Intel chips. Not Intel-made PowerPC chips, not 64 bit Intel chips but run-of-the-mill 32 bit x86 chips, Pentium 4s and such. Apple to Use Intel Microprocessors Beginning in 2006
Why am I freaking out? Mostly I think it's irrational, an indicator of how far down my brain stem Apple has managed to insert itself. The rational part of me says it's because Macs are objectively good ("oh sure, but look who's telling me that!") and this third major transition could jeopardize its future. Apple's fortunes took a major downturn during the 68K->PPC transition but it's not clear that the transition was really to blame. It is the OS that counts more than any other component of the Mac so in that sense the OS 9->OS X transition was much more risky. However with that transition there were so many advantages to be had.
What will switching to Intel do for Apple? Yes, overall performance will be better but unless there's some more major news, overall performance gains will be at the expense of the specialized performance of features like Altivec. I think the much bigger advantage is the elimination of chip shortages hurting Mac availability and therefore sales. Who knows how many sales have been lost due to such shortages? I bet Apple has a pretty good idea.
The computing environment is becoming a little more homogenous. It seems particularly unfortunate that the x86 architecture, which seems to have so much historical baggage, is even more solidly the standard.