Yeah it's a difficult one, our one experience with doing it "after party" was that hardly anyone bothered to check the shots. If I could provide prints on the day it might be different but I will not be offering that (too much stress and hassle especially as I act as disco "roadie" too!).
But will people pay for the party photo service upfront? What would they pay? Not sure - I do have a bit of a portfolio now to show people (provided I can get a gallery site working lol) but normally birthday parties etc aren't places people think of photography. A wedding reception might be a bit different but then they might have booked a pro already.
hmmm lots of food for thought.
I have sent the photos from the last wedding reception off on CD for the bride, but as she's on honeymoon I've yet to get feedback, but so far the other people I've shot at parties have been pleased with the photos. Also my friend was very pleased with the ones I took of her son's 4th party, I asked £20 just to cover fuel and a few minor costs but she gave me £50 on trust before seeing the pics as she'd seen my photos of her and her mum's parties before so that was gratifying!!