Hi Toby. According to Answers.com and Wikipedia, start-up is the correct way to spell it. Now I am sure there is a difference between American and English spelling though. I have seen it both ways; startup and start-up. According to Overture, 3538 searches for "business startup" and 9049 searches for "business start up".
Also, when you do a spell check on Outlook for startup, it says start-up is the correct spelling. Very similiar to home based business or home-based business. Anyone else want to chime in with their thoughts?