Guy Ritchie has a tendency to cast the same people throughout his films. That's not a bad thing by any means; Ritchie often finds superb actors for the job that fit right into his genre of movies. Jason Statham has appeared in three of his films, and soon, Mark Strong will have too.
Ritchie fans know Strong as Sorter in Revolver and Archy in RocknRolla, but the less initiated probably know him from films like Stardust, Babylon A.D., and Body of Lies. Sherlock Holmes will bring him his biggest role yet, however, as the primary antagonist to Holmes, the Satanic Lord Blackwood (described by the poster as a "depraved adversary."
So far, Strong is the only actor from previous Ritchie films slated to make an appearance; however, I'd bank on a few cameos from old favorites. (Vinnie Jones, anyone?)