The multiplayer server I am playing on is using alpha version , because beta dosen't add anything too useful (throwing eggs? really?, oh please) but it does lower frames per second. So the question is the same, how do I log in AND keep mc from updating at the same time?
There's another post on here where somone created a "spoofed" minecraft.jar and minecraft_server.jar which do not update. I suggest looking that post up and applying it to your server/client to prevent it updating.