Err, at the very least, on en.wiki, he's the sole member of the "founder" class.
That's just a courtesy title, as far as I know, there are no rights associated with that user group.
Perhaps his only remaining 'de jure' authority is to appoint or de-appoint ArbCom members.
"de jure" under what law and enforced how?
As for open rebellion - not without the foundation's support. Can't just occupy the servers. ;)
We wouldn't need the foundations support, we would just need them not to object (and if they do, there's always the option to fork, but things would have to get seriously bad for us to even consider that). Jimbo has no direct power over the servers (he has a single vote in board meetings, that's it).
You could always run a strawpoll.
I have no desire to oust Jimbo, so I'm not going to start any polls on the matter. If anyone else does want to oust him, they are, of course, welcome to try.