my impression is that there is a small difference, but i might be wrong.
take these sentences:
'i heard him play his guitar'
'i heard him playing his guitar'
the first sentence seems synonymous to 'i had a chance to listen to him play' (i went to a concert and all.
the second one implies: 'i happened to hear his guitar - i'm sure he's upstairs'.
another pair:
'i saw them cross the street' = i watched them, intentionally, get from one side to the other.
'i saw them srossing the street' = i looked out of my window and caught a glance of them in the midst of that action.
and that's exactly what both doughnut and sobar skillfully pointed out above. i'm just adding 'intentionality' of the observer in . (no, mg, i'm not sure my 'formula' is written correctly for these days i lack linguistic background and rely on intuition :))), btw, is mg mia? haven't 'seen him' in a while!