Although this is a bit of idle banter, Sean Connery has said in an interview that he wouldn't mind playing a Bond villain.

That said though, there are two big things standing in his way, his desired pay cheque, and the fact that the producers would probably never involve and ex-Bond in the film in any other role. However, it's a nice thought isn't it? For nostalgia sake at least.

The comment goes something like this:

“I wouldn't mind coming back as a Bond villain. But I don't think they would pay me enough...They don't pay the money for other parts, only for the Bond character, although that wasn't the case when I was doing it.”

Yeah, the same reason you're not appearing in Indiana Jones again. However he did have some nice things to say about Bond, so maybe he really does want to come back? The comments from from My Weekly through Digital Spy:

“I think [he's] a terrific choice. I think they're going back to a more realistic type of Bond movie as I don't think they could have gone much further with the special effects.”

I really don't see Sean Connery making it back into film though, and I really don't see him making it back to Bond, especially as a villain. That said, it would be a nice nod to the series, but I can't see it being anything but contrived and corny on screen, and would Connery want to be bettered by Bond?