If you join random servers, be aware that they can then impersonate you.

http://www.sk89q.com/2011/09/minecraft-name-spoofing-exploit/

"There’s this interesting exploit in Minecraft that lets you login under someone’s name without ever needing to know the person’s password. All the attacker needs to do is get you to join his/her server once. This client-side fix patches your game so that it won’t let your server tell you to authenticate against a “blank” server ID. Lymia and I reported it to Mojang a while ago, and while Jeb just put a fix in 1.8, there’s a mistake with the fix. You can download a ZIP to install it like any other mod (put the files into minecraft.jar), or Windows users can use the setup program to automatically install the fix"

Ouch, thanks for the heads up. You won't really need the fix if you don't go to other servers correct?

No you wont. We're running the most recent bukkit which has a fix built in, so I couldn't do it even if I wanted to.