Like many people here, I fantasize about working less and devoting more time to my own projects, and a part time job seems like the best option for me.
The difficulty is actually finding one. From what I've seen, jobs seem to either be full time or contractual; not much middle ground.
Should I interview for a full time position and then surprise them with my intentions after I get accepted?
A better approach, if you can take the time to go through it, might be to work full-time for a year, and prove your worth many times over to the company. At that point, ask if you can move to working part-time. They'd likely rather keep a good programmer on staff part-time than let you go entirely.
In my experience, part-time programming jobs tend to pop up more frequently at research universities than at corporations. It may follow as well that a university would be more flexible in turning a full-time job into a part-time job, if asked.