I think the Zac Efron thing is a rumor. E! News had an interview with him for HSM3 and he said he didn't know anything about it until he was asked by the press...so I don't think it's for real. At least I hope not.
Although you never know, because Disney has a habit of using their own already contracted people...and because he's so popular with the younger set, there will be lots more marketing possibilities for them to cash in.
I think I could maybe handle him being in it, but I swear to you all - if there is just one Jonas Brother anywhere in that film, I will be boycotting. And maybe protesting in front of the theater.