This is the first compelling reason that I've heard to upgrade to OS X Lion when it will be released. If this is done right, it will be even more of a killer feature than Quick Look or Time Machine.
Repository functionality on the file system level with a good(!) AND friendly(!) user interface really is something that lots of people have been waiting for since... well, ever, actually. This could be really, really awesome.
I very much agree! I thought the initial Lion demo was fairly 'fluffy' and full of eye-candy but nothing that struck me as genuinely useful.
But version-control integrated into the OS? This is excellent. I wonder how the people who made the excellent Versions subversion client feel about it though.
Of course.. all this fades into nothing compared with the big Lion change: resizing windows by the edges. If they move the close button to the top right corner, then the world is truly about to end.