There is a very, very old White Dwarf article on the subject of running Discworld with D&D. I don't think that I have that issue anymore, I want to say that the number had only two digits though.
Just finished reading Thud! myself (first time around) and it drove home the fact that I really love Discworld trolls. If you look around the Giant in the Playground forum you can find several d20 versions of various races and creatures including the trolls of Discworld.
There's also this site if you're interested in playing the game Thud! from the novel. Nice folks.