/boot is readonly file system -why?

[glenn <gdavy@tpg.com.au>]

Hi - I'm trying to install kernel-image-2.6.4, however along the way
dpkg complains that the boot directory is read-only (/boot=/dev/hdb1 and
/=/dev/hdb3) like so:
--------------
Unpacking kernel-image-2.6.4-1-686 (from
.../kernel-image-2.6.4-1-686_2.6.4-1_i386.deb) ...
dpkg: error processing
/var/cache/apt/archives/kernel-image-2.6.4-1-686_2.6.4-1_i386.deb
(--unpack):
 unable to create `./boot/config-2.6.4-1-686': Read-only file system
---------------
Can anyone tell me why my boot filesystem might be read-only, and what
can I do about it?
For what its worth ls -l shows both /boot and all its contents to be +
rw

Thanks 
Glenn