Apple recently released a small update for Time Capsule users, which tackles an issue where backups made on OS X 10.5 are unavailable for people who upgraded to 10.6. The update does not really fix much, but instead will just verify the current backup and then start you over with a new backup disk image. UPDATE: Apple does not make it very clear that this update will remove your backup history and start you over with new backups. Keep this in mind if you apply this update.
Problems with the backups being unavailable may occur on OS 10.5 machines as well, so people with this OS who have these problems should consider following the steps outlined in this knowledgebase article to verify the backup disk image: http://support.apple.com/kb/HT4076
Keep in mind that all the backup update will do is verify the current disk image, and then start you off with a new one if there is a problem; however, if you are running your backups just fine then you do not need to do this. Do check the accessibility of your backups before applying this update, and if you are able to browse through and access your files just fine, then you do not need to apply the update.
If you decide to apply this update, you might consider taking the precaution to backup the Time Machine image (copy it to a spare hard drive) so if an error happens during the image verification you can restore your backups and be where you left off.
So far, the update appears to only be available through Software Update.