It does not necessarily have to be like that!
Back in times on Amiga a small company called phase5 produced Dual-CPU boards (PowerUP ->
http://www.amiga-hardware.com/showhardware.cgi?HARDID=225), blending 68k and PPC (603/604) together into one system. However, the AmigaOS back then was fully 68k-based, so a lot of so-called "context switching" had to be made in order to make use of the PPC. But it did work, allowing e.g. to playback mpg-Video on a 68040/25 System (the decoding routines running on the PPC).
Warping back to present time, Apple could e.g. blend PPC (970xx) and Intel together into one system again - the systems with 2 CPU sockets are already there (Apple would just have to replace one PPC socket with an Intel one). The difference to the PowerUP cards would be, that with a system like OSX (Tiger) - which is already designed for multi-threading/-processing - the 2nd CPU could be integrated seamlessly. Imagine OSX running "classic" PPC-apps and Windows (apps) at the same time, each one running on their "native" CPU.
No emulation necessary!
But besides that - noone said that Apple will in fact introduce Intel chips all over their product lines. It could also limit the introduction to e.g. Powerbooks or a new device (new Newton, anyone? ;-) first, thus allowing for a _slow_ migration (pun intended

.
And one more thing...
Looking back each time that Apple introduced "standard PC hardware" (Ethernet, ATA, Memory, DVI etc.) some people were afraid everything would now turn worse, but in the end the only impact was, that people could now go and buy standard accessories (Mice, keyboards, memory, harddrives etc. etc.) in the shop around the corner, which also allowed for lower prices.
So if Apple would now introduce the possibility to use either Intel or PPC, it could win more potential customers and have more competition on the supplier side (which is a good thing). And who knows - maybe it turns out that customers prefer Intel-based systems (meaning x86 when writing "Intel") over PPC, because the price-performance-ratio turns out to be better.
I'm totally relaxed on that one, awaiting calmly the things to come (in the worst case my 1.3GHz-Cube can easily serve me another year =)
Cheers
Neodym