This isn't exactly true. I've replaced so many hard drives with various PS3's, it's not even funny. The way it works is that you can swap hard drives with the exact same model without any fuss whatsoever. If you put the HD into a different model, it will complain and ask to wipe your drive.
If you use the backup utility, it will back everything up to the external drive. If you try to transfer the files back, unless you're writing the data back to the same model of PS3, you might not be getting everything back. Singstar songs, downloaded games, and even some game saves, (I think a lot of Capcom games are like this) prohibit transfer to a new system.
The good news is you can re-download any games you bought. The bad news is Singstar Songs (and maybe Rock Band and Guitar Hero songs, but I really don't know) will only come back if you call Sony and beg. As far as the game saves go, you're SOL.
I haven't been able to find a way to crack the backup files or the files on the PS3 HD to be able to transfer them to a different model, but I didn't really spend a lot of time trying.
You may have to deauthorize your machine to re-download some PSN purchases. You can do that in the XMB over by the user settings area.