Experiences seem to vary from server to server, so don't panic if you're not able to log in. You're not alone, and the problem's not on your end. We'll provide updates here and provide you with news as it develops.
Update (12:30 a.m. PST): It appears that XP gains have now been turned on. Happy leveling!
Update (12:54 a.m. PST): Blue poster Bashiok has confirmed on the official forums that XP gains are on universally, so long as you read "not" as "now." The official Blizzard customer service Twitter account says XP gains are active as well.
World of Warcraft: Cataclysm has destroyed Azeroth as we know it; nothing is the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion, 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.