This is my opinion from playing the game once, just now, and without checking the frame rate or anything else which would be considered proof.
I put all the settings on maximum, to see how well it handled, and joined a public server. Even with everything on max it seemed to run well. The only problem was some lag at times [but that could have been my connection (I'm using 4mb/s cable over wifi, btw), someone elses' connection, the server etc.] but aside from that it was lovely and smooth.
I'm going to try again with the settings on medium to see if it stops the lag.
Temperature was about 67C after I'd finished my 20 minute (or so) fragfest.
All in all I was very impressed, especially considering all the horror stories I've heard about regarding other games on Macbooks.
UPDATE - I tried at work over the LAN, there was no lag and it ran better than my G4 with 2gb of ram