I thought about this a little last night and worst case would be if an installer was thinking about heat in AFCI breakers and he got creative in his shiny new (2008 compliant) 60 slot panel.
So he wouldn't put two next to each other, skipping every other space. I would hope someone would catch that but who knows?
If you did want to follow that strategy, checkerboard them. The ones on the odd side, one space off of the ones on the even side. That would give you plenty of breathing room and still balance the load.
BTW I couldn't get NFPA to admit there was a heat problem with a panel full of AFCIs so that may not even be an issue. I guess they never saw a panel in an un-air conditioned garage
