Force delta transfers on rsync?

[Brian Lindholm]

Is there a way to force rsync to perform delta-transfers even on local 
drive-to-drive copies?

The reason I ask is that I've started using a USB flash drive as a 
medium for "poor man's off-site backup".  Rsync does a beautiful job on 
avoiding overwrites of unchanged files, but it would be even better if 
it only rewrote the portions of large files that had actually changed 
instead of resending the entire file.  [Mail files are probably the 
largest culprit.]  Flash memory will eventually wear out over time, so 
the fewer the writes, the better.  Doing delta-transfers would help.  
Heck, it might even be faster, given that flash reads are faster than 
flash writes.

Brian