I'm using the exact same setup as Dan. With the crop factor it works out to 31mm. The new Zeiss 15 is said to be stellar but whoo-whee pricey. You might try a Nikon 20, its supposed to be decent, but imo the ZF Zeiss 21 is the way to go.
Absolutely don't waste your money on one of the Leica M WA's, according to testing by Lloyd Chambers it would not be pretty! His is a pay site, but you can read a little about it on his blog if you go back several months.