Several people have been confirmed to be involved with the 50th, who had previously claimed that they'd not even been approached, or had offered to participate but been shunned by TPTB.
And since their denials of involvement turned out to be subterfuge, pretty much all such denials are now a tad suspect.
Granted, I'd still be a bit surprised if Eccleston showed up, but I'm still banking on David Bradley, and/or Reece Shearsmith, and/or Mark Gatiss putting in appearances in the ep as the Doctors they'll be portraying in "An Adventure in Space and Time".
Likewise, I won't be even the tiniest bit surprised if Sylvester McCoy, Colin Baker, or Peter Davison shows up. And after seeing present-day Tom Baker back in his Doctor drag on the cover of the latest Radio Times...
...I seriously think he looks enough like his old self that he's really just a curly wig away from being able to reprise his generation of the Doctor. (The man looks amazing for being a hairsbreadth this side of 80!)