I ran the classic "PlatinumVerb" test.  For those who don't remember 
this test from the Logic 4 days:
1) Copy a very short WAV file to a track (the idea is that it should 
take up as little processing as possible)
2) Fill the Inserts with PlatinumVerbs
3) Put Logic in cycle mode and start playback
4) If Logic doesn't stop with an overload error, start again on 
another track
5) If Logic does stop, start removing PVs until it will play.

Thanks to Logic Pro 7's Channel Strip save and load feature, this 
test takes only a few minutes to set up and run.  It's not all that 
"real world" valuable--who needs 50+ reverbs in one song, and I think 
this tests floating point processing efficiency more than CPU 
efficiency--but it does give at least a ballpark benchmark using only 
a plug-in theoretically optimized for both machines.

I tested my new MacBook Pro Intel Dual Core 2.0Ghz, 1GB RAM, 100GB 
7200RPM hard drive
Against my PowerMac G5 Dual 2.0Ghz, 1GB RAM, 120GB 7200RPM

I used Logic 7.2, built-in Core Audio driver, 128 samples buffer.

The MacBook Pro ran 92 PlatinumVerbs
The PowerMac G5 ran 120 PlatinumVerbs (23% more)

To put the MacBook's score into "laptop perspective," another Logic 
user ran the same test on his PowerBook G4 1.67GHz and could run only 
28 PlatinumVerbs.