Are there any Common Lisp implementations for .Net?
You could try this (Disclaimer, I haven't tested it myself). Also read this. that's where the link came from.
As of 2019 there is now Bike: https://github.com/Lovesan/bike It implements a cross-platform Common Lisp interface to the .Net Core platform, using lisp compatibility layers.
(saw that on https://github.com/CodyReichert/awesome-cl#net-core)
Reconsidering this question from 2008 in 2010, you now might want to consider Clojure on the CLR. It's not Common Lisp, but it will be fairly easy to learn if you are coming from that direction. Interoperating with the CLR is dead-easy, it takes on more users every day and addresses several other important topics like concurrency. Might be worth investing some time in it. More CLR specific Clojure info here.