From .Mac Help, under "Creating or renewing your membership":
"You cannot change your member name after your account is established."
Of course, you could always change at the end of the year by not renewing your account and getting another one.
Or you could probably call Apple Support with a good story about why your current name isn't going to work (recent divorce, whatever), and they might be able to help you - just be appreciative of their help, but firm.