Right, to play 1.2 if you've updated, I /think/ this is what you need to do:
Go to %appdata%/.minecraft and delete or rename the BIN folder.
Then replace it with the BIN folder you can find zipped all up nice and warm here: http://minemap.tehforum.co.uk/bin.rar
Then launch via the new launcher and choose no to update (if you can log in, I still can't log in via the new launcher.).
I'm off out now, so any problems are your own :C