IncGamers has a great tip for those of you who were on the last Public Test Realm, and can't wait for the 2.3 PTR to come up (and I know you're out there-- you're like PTR locusts!). Make sure to, right now, delete your WoW test realm folders, so that when the new realm comes out, it'll automatically download to the right place and you won't have to worry about anything getting mixed up.
In Windows, apparently Blizzard is recommending that you do a Find Folders and Files (Windows key + F) for "wowtest," and delete everything that pops up. On the Mac, everything is neat and tidy (as usual)-- it's all in the /Applications/World of Warcraft/WoWTest folder. Delete that, and you're good to go for the 2.3 PTRs.
I don't know if this is really required-- it seems to me that the WoW PTR Client would likely just overwrite anything whatever you had in those folders, and your PTR download would still work. But you never know-- maybe files will be left behind if you don't delete, so it's probably best to clear everything out and start over with the new download (and I bet it'll be a big one-- this is a big content patch). Plus, you can feel like you're all ready for patch 2.3: bring on the new PTR!