The new launcher will let you downgrade to 1.2.5. If it's before that, you can try and hunt down the version here. Orrrr, I'm sure there's some way other way to rollback versions.
one thing that sucks with digital distribution is auto patching. i have some levels saved that I last played about 2 years ago. these levels wont work with the most recent version of minecraft, and there is no way to download the old version. i will never see the levels again :-(
it also sucks when steam auto updates a game and all your mods break and you cant downgrade.
Edit: HI PAGE 2!