Problem existed before Leopard...
The problem of Aperture "detecting" inconsistencies in your library is something that happened BEFORE I upgraded to Leopard.
Many other Aperture users have experienced this as well, as you can see here:
http://discussions.apple.com/thread.jspa?threadID=976099&tstart=0
Personally, prior to experiencing this, Aperture has been a really fantastic application for me. I can't wait for version 2.0.
I've had Aperture running while Time Machine has been doing what it does in the background, and haven't had a problem... so far. That said, I'm simply not going to connect my Time Machine external drive to my MBP while I'm running Aperture. All my masters are referrenced on an external drive anyway.
Basically, when Aperture tells you it has detected some inconsistencies in your Aperture Library, it will tell you to hold down Command + Option next time you restart. If you choose "Rebuild Now", all your projects will be gone until Aperture recovers them. If this "just announced" issue causes a similar problem, Aperture hangs and you have to Force Quit it.
I really hope they resolve this issue ASAFP, because it just makes Aperture an unreliable solution.