To avoid messing up the main thread I'll just post this here-
To everyone who is unsure of if the username they want is usable or not, here is the member list. You can search it for the name you want and if nothing comes up then you can have it. Another way is to go to the URL "http://www.pokecommunity.com/member.php?username=[name]" where [name] is the name you're considering switching to. If nothing loads then the username is available.
Keep in mind that members who are banned will not appear on the member list, which is where the second method Razor Leaf mentioned will come in handy. I'm sure that usernames occupied by permanently banned users will be considered as available also, always a good idea to PM an Administrator about it as usual, however.