It was only slightly interesting. The Gutsy kernel behaved in, um, fascinating ways. Something about how it interacted with the console was buggy. This didn't surprise me, really, because I thought I had previously read reports of Gutsy being unreliable on a decTOP. For this reason, I had avoided upgrading while Gutsy was the latest and greatest.
With Hardy out, and being a LTS version even, I decided it was time, and backed up (boot from flash, compress /dev/hda1 to an external hard drive). Using the "server upgrade" process so I didn't have to start X, I then upgraded from Feisty to Gutsy, rebooted with Gutsy's 386 kernel, rebooted with alternate options, and finally rebooted with the old Feisty 386 kernel. (Yay grub!) I then upgraded from Gutsy to Hardy. So far it seems to be OK (when booting with Hardy's 386 kernel), but it never really gets heavy duty use, and I've barely used it since completing the upgrade. The console and lighttpd are both happy, and the boot sequence doesn't show anything disturbing.
I'm hoping that sound will now work, update...it does!
For anyone who wants to try this, I recommend not ever booting with the Gutsy kernel. If I were doing it again,
I'd try removing the Gutsy kernel (2.6.22) before rebooting into Gutsy, to force the Feisty kernel. (The upgrade process appears to avoid removing the kernel you're currently using, but will sometimes offer to remove another outdated kernel if there is one.) Then do "aptitude clean" or "aptitude autoclean" afterwards, to fee up disk space.
I don't actually recommend new decTOPs at this point, except for people really like the ruggedized case or really care about it being as cheap as possible, primarily because the decTOP has only USB1 and no builtin Ethernet, though it does come with a USB-Ethernet dongle. Newer options are out there that have the nice low-power characteristic and actually have USB2 and Ethernet built in. I'll try to edit this entry soonish with one I've heard of (which, sadly doesn't have complete Linux drivers yet).