One of the issues I've seen with startups that do this is that they end up being consumed by the consulting work and aren't really able to crunch on their own thing. Whether it's demanding clients or the cash, your attention gets sucked away from your startup...I've seen people go years without releasing their own stuff when that was their intention from the very beginning, just got caught up in consulting.

But I am very much pro-bootstrapping - save your money while consulting or while working the corporate gig, get your health insurance in line, figure out a method of monetization that starts a good six months before your savings dry up, then quit all outside pursuits completely to focus on your startup.