They might be a little high on meeting room to desk ratio. We had 16 full time and a handful of contractor/intern part time when I was at Bug Labs and we had basically 2 meeting rooms (1 large, one the size of the 5 little ones here) and a workshop space, and it worked pretty well. At most 1 extra meeting room would be good. Having a little extra space for more desks when you expand is a good thing.
I'm not sure how applicable this is to 37signals, but in general I think it is a good idea to separate people who are on the phone all day (sales, customer service) from people who aren't (engineers). We worked fine with 15 people in an open plan, but as soon as we threw in 1 guy on the phone all day it made the open plan annoying at times. And for him if he is in the meeting room to be on the phone all day everyday, why isn't it just his office?
There was actually a tour of our office posted this morning: