There have been a few threads about how to move a time machine backup to a new disk and maintain the existing backups. Some have done this with carbon copy cloner.
This also works with the built in disk utility in Leopard. I turned off time machine first. Then I booted with the Leopard install DVD and ran disk utility. I partitioned the new disk and gave it the same name as the old time machine drive (not sure if that's necessary). I then used the restore function to clone the old time machine disk to the new one. I rebooted, turned time machine back on and it picked up where it left off. The first backup with the new drive took a little longer than expected considering nothing had really changed but all the prior backup data is intact as if nothing had changed.
This also works with the built in disk utility in Leopard. I turned off time machine first. Then I booted with the Leopard install DVD and ran disk utility. I partitioned the new disk and gave it the same name as the old time machine drive (not sure if that's necessary). I then used the restore function to clone the old time machine disk to the new one. I rebooted, turned time machine back on and it picked up where it left off. The first backup with the new drive took a little longer than expected considering nothing had really changed but all the prior backup data is intact as if nothing had changed.