suid not working as I'd hope???

[David Collier]

I have the following set-up

$ ls -l /bin/busybox
-rwsr-xr-x 1 root root 376108 2008-09-07 23:29 /bin/busybox

$ ls -l /bin/date
-rwsr-xr-x 1 root root 55052 2008-04-04 14:22 /bin/date

if I login as web, I can change the date with "date", but if I do
"busybox date" it comes back with 

    date: cannot set date: Operation not permitted

Can anyone suggest why I've failed to get busybox to adopt root user and
do the job?