So, I'm coming at this from a slightly different perspective.
I agree with what both of you (Paul & 6v6gt) say, but I also do small projects and offer ongoing support. I find that you have to go into it with the mindset that you making a commercial offering, not doing it as a hobby. As soon as you take someone's money, you have set expectations in their mind that you have to live up to and their expectations don't always match yours. This is why some programmers on this board make the "buyer" sign a document that spells out what will be provided and when and for how much, etc.
I take a more informal approach for small projects and I provide a limited amount of support before I draw the line. I do this because I realize that I'm often dealing with people not experienced with software and hardware. So just giving them some software and saying "done" won't work for them. If they run into even a minor problem, they probably won't be able to solve it on their own. This is why I have to offer at least a minor amount of support to make sure they get their project running properly. It's also why I price only the very simple jobs for $25 or $50: I'd end up working for minimum wage if I wasn't careful.
For larger projects, I send a document that spells out what will happen when and at what stages I expect to be paid.
I don't want this to sound accusatory by any means, I'm trying to point out that I've seen a lot of what can go wrong and that's why I approach it differently. It can be very hard to be sure that you and the person you're working with are on the same page when your only interface is a computer screen.
I really like working in the Arduino community. The value it offers is huge: I recently wrote a short sketch that took me literally less than five minutes, but it allowed someone to get far more value from machinery that cost thousands of dollars than he was ever able to before. That's the real power of Open Source Hardware and why this community excites me so much.