Yeah, that's definitely the thing to do. I just traded away my 17" pbook for a 12" book (plus cash), and as soon as I get it I will be taking it to the Apple store nearby to slide it in one of those bright orange suckers and see if it fits. They complain somewhat on the store.apple.com reviews about their being some room at the top, so I'll have to see.
I wanted a Crumpler bag for it too, and since this store (EMS) is having a special that ends tomorrow, which is before I get my 12", I took my wife's 14" iBook there and slid it into the bag to see how it was. I figured the 14" would be a decent approximation for the 12" + sleeve. Fit great, and I got it!