You shouldn't be suggesting that idea publicly as it's illegal to download minecraft's .jar file from the internet. Though it's a fine and dandy simple way try to give a more legal response.
Here's one. (Windows 7 user)
1. Right-click your minecraft.jar and select "Previous Versions".
2. If there is one available select it and determine what version it is.
Pretty sure there's other legal ways to get past versions. Just post those before anything else. Something like that can get the site closed down if we're hosted on an on-shore host.