Having converted many databases from SQL2000 via backup and restore and running on SQL2005, changing compatablility is relatively low on my scale of things that need to be changed. I have a mixed bag of compatablilties on my production 2005 boxes and the ones set at compatablity 80 vs 90 perform no differently.

The reason we didn't change them is due to some of the older 3rd party tools they were utilizing. We didn't want to break anything, and once you change the compatiblity you cannot go back.

I agree about the checksum change. That would be higher on the priority list to be changed.