Clubs will never be amenable to a straight salary cap, they'll claim it'll stop them attracting the biggest talent and competing for the best trophies (unless it's implemented across Europe). Atleast, it won't come from the PL - where you need 2/3rds approval (14 clubs, and new entrants like Villa might not like that either). It might come from the Government, but that'll be a drastic step, and I doubt it'll happen in a hurry.
My guess is, if it comes, it'll be similar to a La Liga style - caps based on revenues generated - which will still be a disparity in what the top clubs pay compared to the others. I still doubt it, though.