Try what R C-R says. If no joy, and you haven't already tried it, then Do a repair install of Sno by installing Sno over Sno as in an upgrade install. Such an install, while usually very safe, still is an install so be appropriated backed up if you proceed. You will need to reinstall the 10.6.2 combo updater.
I would not do any sort of install unless I had an external drive with a bootable clone of my internal-drive boot volume on it. I think that there is enough risk in installing that such conservatism is warranted. I would not use time machine for an installation backup; I don't trust it enough.
If you do not get a good install, then you can restore your internal back to where you started using the clone, and nothing will be lost. In doing a repair install, the clone is your data backup and fail-safe device. You can clone with Disk Utility, SuperDuper, or Carbon Copy Cloner. If you like the idea of cloning for a backup, etc., then I would acquire one of the last two mentioned programs.
To clone using Disk Utility (**)
1. Start **;
2. Make sure you external drive is partitioned GUID;
3. Choose Restore;
4. Drag your internal volume to the source box;
5. Drag your external partition to the destination box;
6. Choose Erase Destination; and
7. Click restore.
It will take an hour more or less. Once the clone is made, start up from it to make sure it is functional.