I'm pretty sure you can't undo a patch on Steam - you can stop a game from patching from the "properties" menu, and you can back it up before a patch (and therefore restore to an un-patched version), but once it's patched you're stuck.
If you're lucky, uninstalling and redownloading it from scratch from Steam might get you the previous version if you immediately disable updates, but I suspect you'll get the new patched version.
Thanks for the warning though - I was downloading the patch as I typed this. I have now stopped the download (by setting it to not update) and made a backup just in case I don't like it.
EDIT: Oops - seems that stopping the update still won't allow me to back it up - it insists the update must finish first. Guess I'm stuck as well. I hope they don't nerf my fireballs too much - I liked being able to kill almost anything in one shot.
Last edited by Trippy; 07/12/10 09:59 AM.