Posting this in order to clear up the many threads about ' ZOMG REALMLISTZ R TEH GONEZ'
Q: Where did the file 'Realmlist.wtf' go to?
A: After the Wrath of the Lich King patches (3.X.X) the realmlist.wtf file is no longer inside the root folder of World of Warcraft. You now find it inside the following directory:
Data\enUS\realmlist.wtf
As well, if you use a different region client, just replace 'enUS' with your region code.
Q: What is the 'Config.wtf' file, and what does it do? Can I switch the realmlist inside here?
A: The new configuration file that World of Warcraft uses on loadup sets many important settings, and imports them into the client. This includes the realmlist settings. However, the realmlist, as well as the patchlist, get automatically inserted INTO this configuration file FROM the hidden realmlist.wtf file. So, if you edit the realmlist/patchlist in this file, they will merely be overwritten with the default information when you start the client up, if you have not changed the 'realmlist.wtf' file first.
Q: Why do I have 2 realmlist files? One in my root folder, and then the one in the enUS folder?
A: Depending on the method that you used to upgrade to WOTLK, sometimes the method can glitch up and not remove the old realmlist.wtf. The one inside your root folder is powerless and does not do anything. Just ignore it, or even delete it.
Q: (new!) I've heard a rumor people with a US client can play on the EU realms. Is this true?
A: Yes! Confirmed very recently, the US / EU clients, after patch 3.0, are now cross-compatible. If you change your realmlist.wtf file to match the EU ones, you can then play on the EU realms. And, this also works the other way! EU players can now play on US servers merely by changing the realmlist information. Thank you Toxicity for bringing this to my attention!
I hope this has been helpful. Perhaps a sticky, so we could make sure everyone reads this.
--Information gathered by: Hellgawd
--Q1: Thanks Hellgawd + Noobcraft
--Q2: Thanks Hellgawd
--Q3: Thanks Hellgawd
--Q4 (new!): Thanks Toxicity12