Jason Cohen does a great job outlining a number of reasons why consulting can be a deadly trap for budding startups - namely, that it's hard to get the math right, and easy to end up not making the kind of profits you thought you would make (which then takes resources away from your product).
It's always hard. Most consulting companies don't make much profit, and it's one in a thousand that has the discipline to launch a successful product during off-hours. If you're going to make it happen, you yourself need to be serious, disciplined, and relentless.
Jason also outlines some concrete ways to combat those issues. If you're thinking of using consulting to fund your startup, this is a must-read.