Ok poking around on the net I find this useful document:
It states that Voodoo Banshee is indeed supported by DRI. Working my way through it I discover that I don't have bus mastering enabled (what ever that is...). When I follow the instructions in the doc to enable it, nothing happens. Ie. bus mastering does not get turned on.
Some more relevant output...
lsmikkel@smallpox:~$ lsmod | grep tdfx
tdfx 3840 1
drm 58004 2 tdfx
Device section of /etc/X11/xorg.conf
mikkel@smallpox:~$ xdpyinfo | grep DRI
mikkel@smallpox:~$ xdpyinfo | grep GL
Identifier "3Dfx Interactive, Inc. Voodoo Banshee"
Option "UseFBDev" "False"
mikkel@smallpox:~$ lspci | grep Banshee
0000:00:05.0 VGA compatible controller: 3Dfx Interactive, Inc. Voodoo Banshee (rev 03)