When Apple revealed Aperture 1.5.6 earlier this week, I was ecstatic that Aperture was now ready to be installed on the new big cat. Sadly, Apple's knowledge base states you should not to be running Aperture whilst doing automatic backups with Time Machine as you risk "inconsistencies in the Aperture database." as a result. Running Time Machine restore operations also can cause this issue, but it's worth noting that these only occur whilst Aperture is running.
Apple currently recommends folks turn off automatic backups with Time Machine and revert to manual backups and "Avoid performing either backups or restores while Aperture is running.": hardly the solution of choice, given Apple's pushing of Time Machine as an automated backup system. Either way, I'd imagine we're likely to see another update to Aperture and/or Leopard pretty soon.