Okay, so it looks like it only updated modprobe and not a kernel (see, there is only one kernel listed, 2.4.27).
So what I would try now (though I'm not sure it will work), is to mount the drive like you did in Ubuntu. Then do chroot /mnt/debi
(assuming of course, you use that same mount point). This will make you root in your Debian partition (i.e. it's like being logged into a Debian root console, you are no longer in Ubuntu).
Then you have a couple of options:
1) if you don't mind running Sid (unstable branch codename) totally, then edit your /etc/apt/sources.list and change every "stable"/"sarge" to "unstable"/"sid" (you can use either the branch - stable or unstable - or the codenames -sarge or sid -. Then run aptitude update && aptitude dist-upgrade
. The downside to running Sid is that you'll get a lot of updates and things will occasionally break (about as much as any other bleeding edge distro like Fedora). Sarge is rock solid stable. Nothing should ever break. Or you can use Etch (testing) which will become the new stable in a relatively short time (next couple months, I think). Testing is a middle ground, more stable than things like Fedora, but not up to Debian's "stable" (which means it has to be rock solid on all the CPU architectures supported by Debian).
2) Just update the kernel and see if that will fix it. Do aptitude install linux-image-2.6-386
and see if that will let you boot.
3) Reinstall. Maybe use the new Etch
installer, which will get you newer packages, but not the latest.
You can always upgrade to sid by changing the /etc/apt/sources.list file as mentioned above. That's one of the powerful things about Debian. I can upgrade releases each time without any problems. Just change the sources and do a dist-upgrade.
Let me know if any of these work. I've never tried running aptitude from a chroot environment, but I don't see why it shouldn't work...