Is there a way to install/hack iMessage onto Lion/10.7? My machine (MacBookPro2,2) doesn't support 10.8 and I would rather hold back on doing the upgrade (possible with hacks). So is it possible to stay on 10.7/Lion and get iMessage working?
Since the Messages beta has ended, it is no longer offered for download on the website however the download link direct to the .dmg still works:

Messages beta direct download.

I don't expect this to continue indefinitely.

A more permanent option would be to try and copy the Messages.app from a Mountain Lion install on another computer to your OS X Lion applications folder. Let us know if that works for you.