The G5s are NOT twice as fast as equivalent PCs. Third-party tests generally conclude that the G5s range from "almost as fast" to "a decent bit faster" depending on the task, but they're not THAT much faster. Maybe for one or two very specific Photoshop filters or something like that. But for general use, no way. The best you can say is that they're about the same, really.
I agree, however, that going strictly by what's public knowledge, the G5 is holding its own very, very well. Since its release 2 years ago, the G5's clock speed has gone up more than the P4's.
That said, Apple isn't going by "what's public knowledge"; they know better than we do where IBM and Intel are heading. Steve Jobs made it sound like the big advantages of the switch are coming 2-3 years down the line. I don't know exactly what those advantages are, though.
The best guess I've heard for the long term is that Apple wants to get onboard Intel's hardware-based DRM train. I don't know a whole lot about this, but it seems likely. If Apple's doing this for shorter-term advantages, it's probably because the G4 is stagnating, and the G5 just isn't making it to portables. From what I can tell, even the current G4s compete pretty well with Intel's mobile chips, though, so again I feel like Apple's seeing a bigger difference down the line than exists today.
In any case, it's not because today's G5 can't compete with today's P4. But I have to assume that Apple knows what they're doing. This isn't a move they'd make on a whim.