Since Leopard I’ve been getting “Preparing backup” messages that have lasted forever whenever I try to backup my system on to an external volume (i.e. my Time Machine disk).
Digging around in the system messages, I found this: “waiting for index to be ready” being repeated endlessly.
The fix appears to be to add your Time Machine disk to the Privacy settings of Spotlight so that it doesn’t get indexed and then reboot.
Note: I had another issue adding my Time Machine disk to the Privacy settings of Spotlight which is discussed here: