It doesn't mean OS X running on your Dell PC. There's a lot more to a PC-compatible machine than just the CPU, and I can't see Apple opening up in that way. Don't forget, they're a hardware company. There's also the problem that Apple won't particularly want to write device drivers for all of the PC hardware out there. Having said that, this will certainly make it easier to write a virtual machine that will run OS X under Windows or Linux, and I'm sure someone will do that.
It doesn't mean Windows running natively on your Mac, either, although again it should make virtual machines like Virtual PC faster and easier to write.
It doesn't mean that a PPC-based Mac will become obsolete in 2007. It seems almost certain that virtually every application will be released with a fat binary that will run in PPC or Intel environments for years after the switch is complete. If you were going to buy a new Mac, there's no particular reason to change your mind -- there'll be a faster one next year, but you already knew that.
It does mean that you should think carefully about new peripherals. Device drivers are less likely to run in their PPC-emulator on Intel, and a scanner that you buy now may never work on an Intel-based Mac. Disks, mice, keyboards, etc. are unlikely to cause a problem -- it's scanners, webcams, graphic tablets, non-mainstream non-Postscript printers and weird stuff like biometric verifiers that I'd worry about.
In the end, though, it's not as big a deal as it's made out to be. New Macs in 2007 will run faster than existing ones, and they may not work reliably with 100% of the software and hardware that is available at present. What about that did you not know before Monday?