Seems that the Actor Fund is having auctions to raise money for their cause. Two of the auctions will have the winning bid appear onstage on Broadway. Two shows have a bid going, Mamma Mia and Rent. Check it out:
Link to Mamma Mia Auction
Rent Auction
The Mamma Mia thing is for a walk on during the wedding scene. The Rent auction will actually have you singing onstage. I don't think I would be cruel enough to let thousand of people hear my horrible voice.