Mila Kunis has a little nastiness that just doesn't work for Mary Ann, IMO.
Rachel Leigh Cook, though? She'd work.
Here's the thing, though. If Hollywood really DID redo Gilligan's Island you'd get something idiotic like this:
Adam Sandler or David Spade as Gilligan
Cedric the Entertainer or Kevin James as Skipper
Drew Barrymore or Lucy Liu as Mary Ann
Kirsten Dunst or Maggie Uglyassinhall as Ginger
John Travolta or Owen Wilson as the Professor
Nicholas Cage or Kevin Costner as Mr. Howell
Goldie Hawn or Susan Sarandon as Lovie
It would suck tremendous ass.