I don't know how common this feature is but as well as transposing the score, Cakewalk can transpose one or more parts for playback. I have not tried to use it for clarinet transposition but I have found it useful when writing for instruments like the tenor banjo which play an octave lower than written. A slight drawback is that the files need to be saved in Cakewalks own format as saving as a MIDI produces a score written as it sounds. Another drawback is the price. CakeWalk Home Studio 9 which I believe is thier basic program is currently US $129.

Jon