Good news for all you DB2 for Linux people who love Ubuntu -- IBM just validated Ubuntu as a Linux distribution on which IBM is willing to offer its enterprise level support. You can see that Ubuntu was excited in their press release about the accomplishment. And rightully so: no other database server has announced production-level support for Ubuntu. Cool.
I've issued some updates to the DB2 for Linux HOWTO that will hopefully be published soon; I had based the original Ubuntu install instructions on my Gentoo experiences and there were a few embarassing references to that that I hadn't cleaned up (and which were painfully evident now that I'm running Ubuntu 5.10 "Breezy Badger" on the dual-boot side of my laptop).
In a future update to the HOWTO, I plan to offer a little more fine-grained installation advice; the current instructions install everything under the sun, including support for 29 different languages, warehouse agents, and assorted other features that probably aren't required for most developers or users. But at least you can get up and running relatively easily
So if you want to try Ubuntu, go to http://ubuntulinux.org. It has replaced SuSE Workstation 9.2 on my laptop -- and I'm even using Gnome again, which I had given up on a few years ago.
If you want to try DB2, you can download a 90-day trial of DB2 Express.
IBM typically doesn't actually perform the work to validate DB2, it assesses the results of the validation testing performed by the partner that wants to earn the validation. But I can see in my post that "IBM just validated Ubuntu..." makes it sound like IBM does all of the work. Ah, the vagaries of the English language!
If Ubuntu wants to be validated as a Linux distribution for running Informix IDS, then they (or more likely Canonical) would have to take the initiative in contacting IBM. And they're not likely to do that if there is no demand from their users -- so I would recommend that you let Canonical know that you would be interested in running Informix IDS on Ubuntu.