It really does not matter much if you are doing it part-time or full-time except for the time it takes to get the first release out. I have been working on http://www.onista.com for almost a year now and I am doing it mostly in evenings and weekends. Bear in mind that I have one and half year old kid who takes lot of time away from me (Off course I enjoy it a lot). Recently two more friends joined me and we are pretty confident on launching in November.

Based on my experience I can say that, persistence is what matters. If you work full-time and are not persistent or loose focus then you are not going anywhere. If you work part-time and are persistent and focussed then you will win. May be it will take more time than usual but you will get there with better results.

So I would say, yes go ahead and work part-time on your idea, but be very persistent to spend whatever extra time you find on the idea. It would be good idea to find group of friends who can work part-time with you. (I did that it does help a lot)