Unmounting a busy RO-Filesystem

[Bourne]

I have 3 partitions. /dev/hda3 for '/' , /dev/hda1 for /boot and 
/dev/hda2 for swap.

I boot & then i do a CTRL+ALT+SYSRQ+U.  '/' and '/boot' are now 
remounted ReadOnly.

1) cd '/boot'
2) umount /boot ----> This gives me an error "Device Busy"
3) cd /
4) umount / -------> No error
5) echo $? -----> outputs '0' indicating success. !!!!!!!!

When i do the above by skipping the Sysrq part, i get the usual expected 
errors.

So is the above behaviour expected ? I tried this on 2.4.19 & 2.5.51. 
Same results.

I am subscribed to the list

TIA
Bourne