I've written about this several times on my blog "Up and Running" here at Entrepreneur.com"
Ideas are Easy, Doing Stuff is Hard
Also, you'll like this one, on my Planning Startups Stories blog:
Second or third mover advantage
It's answering this same question, although it was a while ago ...
Tim Berry is the chairman of Eugene, Ore.-Palo Alto Software, which produces business-planning software. He founded Bplans.com and wrote The Plan-As-You-Go Business Plan, published by Entrepreneur Press. Berry is also a co-founder of HavePresence.com, a leader in a local angel-investment group and a judge of international business-plan competitions.