We can only guess for now.
My best guess is that all PPC apps will be slower on these new Intel machines than on the machines they replaced. That's pretty safe to say.
There are several kinds of apps to consider, though, and the performance will vary. Apps optimized for G4s will suffer a greater performance hit, because Rosetta emulates the G3 only.
And I have not heard anything about if/how Rosetta handles multiple processors/cores. It's possible it'll use the two cores on the new Intel chips just like it would on a Mac, which would help the Intel Macs make up the speed difference, since the Macs they replaced had only one core each. But it's also possible it won't use the second core at all. After all, there's no such thing as a multi-processor G3 system. Also, most apps that are really optimized for multiple processors/cores are optimized for G4s and G5s, so I'm not sure how that would translate anyway. Time will tell. My guess is that Rosetta will use the second core; however, it's possible apps like Photoshop are written to use single-core routines when they detect G3s, which would make the point moot. Again, time will tell.
Bottom line: We'll have to wait until users and review labs get their hands on these things and do REAL tests. Apple has not offered any benchmarks on this stuff. They've only said that Office runs well (and that's to be expected).