Yes,you can clean tile roofs.Yes,they can be walked on,but very carefully is an understatement.This is not a job for a homeowner to try.
Look in yout local yellow pages for pressure washing and call the companies that list "low pressure" roof cleaning.Ask for an estimate,references and before/after pics.If you can't find someone close to you,email your addy and phone number and I'll see if I can locate someone on pne of teh pressure washing sites.

Thanks Scott.. Your right about walking up there.. That's why I wanted to make sure I got the right guy to do it so I don't get any broken tiles.. I love tile roofs but there such a pain in the butt to maintain.. I have a skylight in the gameroom and if I let it go any longer were not going to be able to see out of it.. lol It hasn't been cleaned up there since we moved in and it's going on a year. I will try your advice and check the yellow pages....
Thanks again