I'm an American, so I don't really know anything about the Proms special (hell, before I saw the article on Wikipedia, I thought a prom was a high school dance), but there's going to be a mini-episode aired during the Doctor Who prom. It's written by Russell T. Davies, and here's this excerpt from Wikipedia:

"According to Russell T. Davies, the action will take place in the TARDIS and will feature "a special returning guest star.""

Who do you think the guest star will be? The popular guesses right now seem to be Jenny and Astrid, and I'm tempted to go with the latter. (Also, a Graske is rumoured to appear.) Thoughts?