Rosetta works by translating the PPC command set into Intel command sets.
It's not an emulator and so there is not that much of a performance hit.
This change is going to be much, much more transparent than the change to
OSX.

I bet Apple is working hard on the semantics of whether it's an emulator or
not. My vote is that it is, and that there will be a considerable
performance hit, but you won't notice since the Mactel's will have twice the
CPU power as your previous PMac, and only Logic will take full advantage of
the new architachture (but then you're stuck using Logic...). And the
transition will be as "transparent" as the introduction of Altivec, which
took over 5 years to come into widespread implementation. Remember working
on early G4's back when they came out? I recall saying "screw this, gimme
my G3 back".