Same thing has just happened with me! Had an email overnight stating my bid had been accepted, and a pending payment appeared on my credit card this morning.

I've just received an email saying "Your recently upgraded offer on your upcoming flight has been refunded".

Called Virgin to double check and they said it has been refunded due to no seats available, so I'm guessing either someone has purchased a full paying ticket last minute or they bulk accept multiple bids and then it's potluck on who actually gets the seat?