iWeb currently only offers Debian Lenny as a pre-install option on these servers, but since Lenny is dead as of February 2012 I wanted to start with Squeeze. I tried the upgrade process from the debian.org site linked below first, but my server didn’t reboot properly after the GRUB2 install and I couldn’t even connect to the VNC console of my VM. Since the hardware is about a bazillion miles away I don’t know if there’s anything I could have done from the console to fix grub and rescue the install; my only option was the auto re-image in the iWeb control panel. I re-imaged the server with Lenny and while that was happening I did a bunch of reading about other people having fun with the new grub.
This is just a quick step-by-step. Basically the standard instructions break grub-pc (GRUB2) on this Xen-based system. Follow along until the end, then remove grub-pc and re-install grub-legacy. Your system will then be ready to go!
All of this information is here: http://www.debian.org/releases/stable/amd64/release-notes/ch-upgrading.en.html but this page is far less reading.
Also, if your system is not a “stock” Lenny install, i.e. with 3rd party deb sources and a bunch of custom stuff, your mileage may vary. I did this to a fresh, new server before anything else.
apt-get purge splashy
### Change all references of "lenny" to "squeeze"
### Need to find our architecture which turns out to be "xen-amd64"
apt-get install linux-image-2.6-xen-amd64
apt-get install udev
### When asked if you want to chainload GRUB2, say "NO"*
apt-get install grub-legacy
### enjoy Debian Squeeze!
*Answering “NO” here tells the installation script to just go ahead and fully install GRUB2 and not mess with the legacy grub .conf file and fart around with chainloading and stuff. I just felt better uninstalling a “complete” GRUB2 install rather than a half-assed hodge-podge of grubbery.