Implications of feeding birds

This is a topic that always interests me - what effect do we have on wild birds when we put out bird feeders?  I worry almost constantly that I'm only helping the invasive House Sparrows and European Starlings when I put out bird food.

There's an article on BBC News (found on Birdchick) detailing the evolutionary changes underway within the Blackcap.

There's an interesting lack of opinion as to whether this is a good thing or a bad thing... Must nosh on these thoughts...