Well this is quite interesting:
http://www.hitfix.com/articles/stephen-fry-will-spar-with-robert-downey-jr-in-sherlock-holmes-2 and here:
http://www.denofgeek.com/movies/612203/stephen_fry_signs_up_for_sherlock_holmes_2.html and I hope it's true! :-)
BTW Part 2 of Stephen Fry's autobiography The Fry Chronicles is now published in massive heavy hardback format. Am waiting for publication of paperback version, but that won't be available until next year at the earliest. It is available for Kindle download now, and will soon also be released - narrated by the author naturally - as an audiobook:
http://www.amazon.co.uk/Fry-Chronicles-Stephen/dp/0141041587/ref=sr_1_4?s=books&ie=UTF8&qid=1285581705&sr=1-4 Should be very interesting, as it covers his Cambridge years when he meets up with Hugh Laurie, Emma Thompson, etc.