After trying to update my kernel for a few times now (and failing because iSCSI never worked after the upgrade) I delved deeper with added vigor due to some SSDs arriving next week and I want TRIM for them! I finally pulled together a number of bits of info and made this easy guide to update the kernel and get iscsi working again.
Hope it helps any in the same position as I was!
1. Edit "/etc/apt/sources.list" and ad the line:
deb http://backports.debian.org/debian-backports squeeze-backports main
2. Update the kernel
apt-get update
apt-get -t squeeze-backports install linux-image-2.6-amd64
reboot after this completes.
3. Install all the packages required for what follows
apt-get install subversion libapache2-svn iscsitarget-source iscsitarget module-assistant debhelper linux-headers-$(uname -r)
4. Clean up src folder and download latest iscsitarget source and compile it
cd /usr/src
rm -rf iscsitarget.tar.gz
svn co https://iscsitarget.svn.sourceforge.net/svnroot/iscsitarget iscsitarget
cd iscsitarget/trunk
make
make install
5.Checks, tidies and bingo
depmod -a
modinfo iscsi_trgt
service iscsitarget restart