I experienced a very strange situation where rsync simply stopped working. It wouldn't even output anything for rsync --help or even plain rsync. which showed it to be in /usr/bin/rsync as expected. A closer look revealed something odd:
$ ls -lah /usr/bin/rs* -rwxr-xr-x 1 fitzsimj None 0 2010-01-11 13:40 /usr/bin/rsync -rwxr-xr-x 1 fitzsimj root 349K 2010-02-19 11:20 /usr/bin/rsync.exe
I had a mysterious, empty rsync file next to rsync.exe in my /usr/bin folder. Deleting the rsync file fixed my problem. I have no idea where this came from.