If you're hoping to somehow get your level 80 character a rested XP boost come Cataclysm, the only way would be to log him or her out at an inn and not play him for a while once launch day comes, because rested XP is being reset with the launch of the new expansion. To quote Valnoth:
You can stop trying to come up with weird scenarios in which you will get a huge rested XP boost before Cataclysm ships. It's not going to happen. If you want rested XP, you're just going to have to rest for it.
World of Warcraft: Cataclysm will destroy Azeroth as we know it; nothing will be the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion (available Dec. 7, 2010), from brand new races to revamped quests and zones. Visit our Cataclysm news category for the most recent posts having to do with the Cataclysm expansion.