How do I remove drip marks in flat paint on smooth interior walls?
If the painting was recent (within say a month or so), and is still a bit rubbery, you can do a great job using a single-edge razor blade to slice off just the offending drip.  Hold the blade almost flat to the wall so it doesn't dig in to the surrounding paint.  Then you'll just have that little circle to touch up with a dab from the corner of your brush.

This also works for other similar blemishes like roller fuzz or small chunks of latex.