Dave Shea asks a great question and gets some great answers. Personally, I have way too many things going on outside of work right now to even think about this, but Dave raised a great question that is nearly always in the back of my mind. I'd possibly rephrase it to say: "Is there something I can create that has me as the client and still have an income?"
The answer is as variable as the number of designers out there. I think the folks over at Coudal have tapped a magical vein for themselves. Carlos Segura of t-26 and 5-inch and the gang at 37signals have also figured out what works for them. (Wait a minute, they're all in Chicago... What's in the water there?)
What works for me will be different that what works for the next person, but we all see the wild success of others and long to quit doing client work at some point. In the end, I believe it's all about having the courage to follow one's passion to see where it leads.