I've heard the same question from a couple of different Internet entrepreneurs recently. Traditional wisdom would say that you should not launch until you are absolutely prepared to make a big splash. After all, you never get a second chance to make a first impression. Get your brand right, get your system right - and, only then, after you've tightened all the screws and double-checked all of your lists, should you launch your new site.
I believe traditional wisdom is wrong.
I think you are better served creating a Web site, a blog, a forum, whatever - to engage your community ASAP when starting your business. Obviously, you have to have a clue of what you are putting up there... but just. The clue is all you need. The market will provide the rest while you continue to build your business plan and operations behind the scenes.
There are a few key differences between Internet Startups and traditional, geographically based businesses. Take a look and see if you agree: