I'm seeing the same problem.
I've got an airport extreme with an external disk. Multiple mac's have backed up to it for some time without issue, including my original mini9.
I just enabled backups for a second mini9 today. At first it grabbed/overwrote the original mini9's backups.
I managed to get them to back up seperatly, by removing the original backup sparsebundle, and telling them both to start backing up at the same time. They each made a backup bundle with their own machine names.
Trouble is a few hours later they'd both switched to using the same sparsebundle.
the bundles are names correctly when they're created, but it looks like thats not what actually connects a bundle to it's machine. The .ethernetaddress file doesn't appear to exist for the new machine which is odd, but there is one for the old mini9. However the old one is now picking up the new machines backups. Are these files still used in snowleopard, as they're all dated as pre-sl, and the new machine which has only run sl doesn't have one?