Aperture users that have upgraded to Mac OS 10.5 Leopard should beware when using Time Machine. According to an Apple support document, having Time Machine run a backup while an Aperture session is in progress could result in inconsistencies in the Aperture database.
Apple recommends that Aperture users set Time Machine to perform manual backups. Apple also says that they will be updating the support document as more information becomes available, perhaps indicating that they are working on a resolution to the issue.