This only applies to Site Collection backups, and not farm backups. Farm backups are apparently just fine as long as you run the psconfig update command afterward. I haven’t had to deal with these specifically yet, and if those have the same issue then I’ll post at a later date (when I come across those issues).
If you have a site collection backup from before a service pack was applied, you can not restore from that backup after the service pack. What you’ll want to do is to set aside another environment (even if it’s a virtual image) from before the update in case you need to restore one of these old backups in the future. After you have the content database restored with the site collection in it, you can attach to the new environment and run psconfig’s update to get it up to speed.
The pre-SP1 environment I stood up for this purpose was a Windows 7 developer install of just Foundation on no domain at all. The final destination was in a domain running server.
Yes, I just had to do this. Yes, it was a huge hassle. Yes, it worked.