--delete bug?

[Joost Ruijsch]

I have a script that does a weekly backup on a local backup harddrive 
with the following command:
# rsync -aRxW --delete / /mnt/backup
Today I got errors because the target drive was full. It appeared 
nothing on the targed drive ever got deleted.
When I used the following command to backup only my home dir, it worked 
as it should.
# rsync -aRxW --delete /home/joost /mnt/backup
Using rsync 2.5.6,  but I have the same problem on another machine with 
rsync 2.4.6. Both use reiserfs.
Does the delete option not work when source is '/' ?

Joost.