As far as I am aware, Steven Moffat promises an 'explosive' and 'special' 50th anniversary. They may throw a big party or something along those tracks. I think we can safely say Chris Eccleston won't be attending. Furthermore, in a convention, Tom Baker said he is not keen on the idea of acting alongside other Doctors. Matt and J-Lou (Coleman) will definitely be there.
Martha, I don't know. :( The team seem to be quite secretive...hmm...
"Dear God, I hope not" was exactly my immediate thought... I really found her incredibly dull, whiny, and overall a thorn in my side of a character. She wasn't snarky enough to be Donna and not compassionate enough to be Rose. She was a try-hard of a companion, in my opinion.
I can understand where the character was meant to go.. but the acting just wasn't there.