I was bitten by the nasty "I'm tracing myself and I can't get out" UML bug on 2.6 Linux hosts. Did some research and found a patch by Jeff Dike which claims to fix the mess. I've already download user-mode-linux source (the Debian package source, I mean) and right now I'm downloading all the needed build-deps. Once everything finishes downloading I'll try to build a Debian package with the patch applied and start playing with UML on 2.6.
I had already played with UML a bit some months ago under 2.4 and found it really useful when I needed to do some experiments and won't afford losing my main box. After going 2.6, I won't go back to 2.4 again only for UML as I'm pretty satisfied with how 2.6 is performing on my box. A working UML is the main thing which keeps me from being completely happy on my 2.6 box.
I everything turns out to work fine, I'll have a lot of work to do and lots of hacking/installing/configuring in order to put into practice all the ideas I have im mind currently. Nice !