If it's oil-based paint, forget it. If it's latex paint, you may be able to soak the stain in cold water and detergent if it hasn't been machine-wased yet. If it's been machine-washed already, you've basically set the stain.
You could possibly 'pick' at the paint or consult your local dry cleaner service. Other than that, the holidays are coming so try and get an 'advanced gift' of new pants. ;-)