Lauren Shuler Donner said they'd never do an X-Men movie w/o Hugh, and I believe Fox agrees with that statement. I'm assuming she meant the current crop of films (with the current cast). If
there's an X4, you better believe they'll get Hugh involved in some way (an extended cameo at least). I assume after Wolverine, Hugh is going to want to move away from the X-franchise for good, though. I think future films (whenever they happen) will move away from the current trilogy storyline, and feature a mix of old and new mutants - which will be a good way to explain Wolverine's lack of screentime (or departure).
It's my opinion that they should bring Gambit into future films to take Wolverine's place as the rebellious character. Maybe that's why they've been keeping him from appearing in these films? Because they know he's also a very popular character amongst fans?