Games Workshop produced a game a few years back called Inquisitor which was a detailed skirmish game designed for 54mm models. I bought into this in a big way and have three or four warbands plus an assorted cast of security troops, mutants, and other NPC's.
Distances in the game are given in yards so it's easy to convert into 28mm as well.Since the default 54mm game calls for a 4x4' board, you can play 28mm games on a 2x2' table with the additional benefit of being able to pull models and terrain from your 40k collection!
For an additional note on 40k Friday, the 6th edition game preview (and pre-orders?) launches tomorrow at a Games Workshop store near you!