Apple made it so the keys just 'pop-off' instead of 'breaking off' so you should be to stick that sucker back on. It may take some patience but keep trying, I've poped off several keys on my iBook but they've all [eventually] gone back into place.
In that case there's something wrong, write an email to Apple and they may just send you a replacement key.
And next time you want to clean your iBook's keyboard, buy a $4 can-o-air and just spray all the crap out from underneath the keys. Removing them is probably okay 95% of the time, but that one time the B key won't go back just sucks too much ass to be worth it.