Some people over at Gearslutz have also reported exceptional audio software performance on the Macbook, but I don't know how many talked specifically about Logic or not. A lot of people will say that their Macbook is handling sessions (some say Logic, some say Pro Tools) that their old dual 1.8-2.0ghz G5's are starting to choke on, so I would say that you can expect at least comparable performance to the current low-end Powermac. Not too shabby.
The thing is, it's possible that integrated graphics will make a DAW with too much eye candy LOOK like it's running chunky, but in reality there is plenty of processing power to keep the software running smoothly. I'm sure that with the RAM maxed out in the Macbook (since giving it more RAM DOES in fact affect graphics performance) you wouldn't even notice any problems.
For the record, I use Logic Pro 7 on my ibook sometimes. I did a small mix on my imac G5 and took it over to school on my ibook to finish, and it still ran fine. I had 8 24-bit tracks going with a couple plug-ins on each (one of which was Space Designer) and the CPU meter was at about 25%. Granted, that's lousy performance, but this is an IBOOK we're talking about!
I was happy that the session even opened!
OK, I'm exaggerating a little bit...