There's some good answers hereabouts too, but overwhelmingly they appear to be taking the employers' perspective, so let me redress that a little.
Firstly, I think it is great that you are wanting to take a better work/life balance than an ordinary 9-6 job will offer you. We who live in advanced capitalist economies often need reminding that our society has created a 'normal' workaholism that takes from most people most of their useful waking hours. You may therefore need a bit of encouragement in your quest, and I would start with recommending How to Be Idle by Tom Hodgkinson. In it he argues for a de-stressed society that lives cheaply, works less, makes more merriment and relies less on expensive/pre-packaged food and entertainment (I believe his How To Be Free is on similar lines, and I forget which one it was that I have read).
I'd also say that you shouldn't mention your part-time intentions until you've got through first-stage interview. You can then ask at second stage interview, or via email/telephone contact, if your application progresses.
Consider also only asking for four days/week. For some reason, I sense there is quite a gulf between three and four - maybe because Friday tends to be a wind-down day, and a good programmer cannot be expected to run at full productivity five days a week anyway. So I think you'd be more likely to get a nod if you start asking small.
Also, would you consider taking a flexitime job on with full-time hours? That would be nine-ish hours per day rather than seven-ish, but would mean that your employer gets the benefit of a full-time developer whilst giving you the win you're after. That of course presumes you'd be happy with just one extra day off per week. Incidentally, some while back I noticed a tasty-looking advert on StackExchange for a four-day week dev based in San Francisco - so they are out there (edit: I wanted to link to it but cannot find it - it is probably gone!).
That last point brings me to my next one: apply to small companies or start-ups. Small firms don't often have the budget to pay good developers handsomely, so they may offer non-remuneration benefits instead. Also, there are less layers of middle-management to get unusual requests rubber-stamped.
My path has been somewhat similar to yours - I used to work for a large company as a PHP/Oracle web programmer, and they insisted on my commuting to the office every day, resulting in 300 miles per week of pointless mileage. I requested flexitime and home-working at various strategic points (reviews and the like) and they were repeatedly "considered" and turned down. I enjoyed the job and the people, but a couple of years later, I gave up asking, found a small UK company that offered both flexitime and telecommuting, and resigned from the corporate job. It is my hope that in leaving over it, I have very slightly improved the chances of others who want to do the same.
So, I wish you the best of luck - if you are successful in this endeavour, you also may help out other developers who feel the same as you do. If you get there, do document somewhere on the web some tips to help others!
Edits: an interesting article and comments on this topic, and another from 37signals. Here is how companies can set up four-day working weeks. Less on-topic but very interesting - remote working by the chap who set up this very website!