Although they only state explicitly that Swank is confirmed, from reading producer Avi Arad's comment over there it certainly seems like Sam Jackson is confirmed as well.
When asked about Hillary Swank's appearance in the film, reportedly Arad replied:
"How do you know [about that]? ... It’s a cameo."
This was followed by:
"The Sam thing was supposed to be the biggest secret of them all... it’s amazing how it got out."
Yes, our spies are everywhere.
Hey Avi, don't get mad it me - I'm not the one who broke either the Hillary Swank or Sam Jackson news. :-)
Source: MTV Movies Blog