I just recently managed to get mythtv to work on a straight up install of debian sarge kernel 2.6 by adding the following to my /etc/apt/sources.list:
deb ftp://ftp.nerim.net/debian-marillat/ sarge main
and then do "apt-get update" and "apt-cache search myth" to see what myth related packages you can install.
You'll need to install a lot of dependent packages but if you select "Desktop" and "Development" environment during the initial install you'll have most of it.
I pieced my install together based on info from various (some out of date!) sites such as:
http://dipper.info/project/ivtv/
www-isl.mach.uni-karlsruhe.de/~hi93/myth/mythtv_debian_epia_pvr350_walkthrough
http://www.quietearth.us/mythtv.htm
It all worked nicely with the installed binaries but I did have to compile ivtv drivers and a few other packages from source.
I've been meaning to write a similar sort of install trail for debian sarge on my hardware (stock Dell Dimension PIII & PVR-350) but haven't found the time...
I'm also not finished yet, I'm trying to get MythGame to work, I need to setup an IR-blaster to control my satellite box and I want to add a joystick.
In addition, I want to see if I can stream live TV or movies with/without mythtv (I've been looking at vlc (http://www.videolan.org)) but so far it only barely works - something needs to be tuned or adjusted somewhere.
Hope this helps!