Hate to bring this topic back...
I've got a SRAM red cassette, and the same Ritchey WCS Protocol wheels as the OP had. I thought I was pretty stupid...couldn't get the splines lined up for about 20 minutes...then I realized that the step in the freehub body was the problem. in talking with MY LBS they said that shimano changed things up recently (probably at the advent of SRAM's road gruppos) to make it harder to use shimano branded items with SRAM...at all. In other news, the cassette does fit on an older shimano hub that I had my 9-spd on, as well as my mountain wheelset...in case I ever want to get a little crazy.