have you tried going to 'layer-matting-remove white matte' or defringe. these will choke your matte slightly
if your lucky enough to have your subject against a primary colour you can use the channel mixer to create a alpha.
Put in in monochrome mode. put the background colour to as low as it will go and then ramp up the constant slider.
then use the result as a layer mask.
This one was quick, you can get more subtle with it
If your feeling fruity you could also try After effects or Shake and pull a key with keylight. hell Max and maya come with a compositor now. they should have good keying features.