After much experimentation with the various known techniques and applications for turning off Spotlight indexing on a volume, I have concluded that it is not possible to convince OS X (Leopard) to not index the volume being used for Time Machine.
This is true even if Time Machine is set to Off, its volume is added to Spotlight's Privacy list, and the volume has a .metadata_never_index file at its root. The volume I was using is a partition on an external FireWire 800 drive dedicated to Time Machine.
My guess is that Apple enforces indexing on the Time Machine volume to facilitate searching backups, as suggested in this knowledge base document. But there are many ways to search, I don't use Spotlight all that much, the volume is 400GB, and I really just don't want Spotlight chewing on it -- especially when there's no way to control the frequency at which Time Machine does its backups.
[robg adds: Note that you can actually control the frequency with which Time Machine backups run, as discussed in this hint.]
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