I've just come from a site where Intego's VirusBarrier X5 was the sole cause of freezing a 10.6 Time Machine backup and the system with it EVERY TIME. Simply by turning off the Real-time scanning, the backups were able to proceed.
So - if you have upgraded, or about to start using Time Machine - I recommend ditching intego's virusbarrier X5 and getting the Sophos Anti-virus for mac Home edition for free - see my earlier post. I installed Sophos and lo and behold, it all works fine!
NB. Issues were found with 10.6.3 and updated 10.6.5