Not sure about "solopreneur", isn't an entrepreneur just a single person anyway :) Although I wouldn't title myself with either.

I've worked on my own since '03, currently in here http://qiip.me/edlea/blogger/post/4019

I freelance mon-fri 9-5 and I build my project evenings and weekends. When I have enough cash I get some help from another developer.

Interestingly RescueTime says I'm ~10% more productive at weekends when I'm working on my project... so I guess I love to work on it, so I do wish I could do it full time.

Motivation (in no particular order): 1. prove to myself I can do it 2. make money 3. help people with a problem that stops them working on what they love 4. [perceived] freedom 5. challenges not in my freelance work

and if you're curious, http://qiip.me is my project.