If you are curious about maybe one day doing your own startup, you should know about a new discussion site called Answers OnStartups. It uses StackOverflow’s software but focuses on the art of starting your own company.
The only previous decent online forum covering that topic is the Business of Software forum of Joel on Software.
Which one is better? I’ll give you the quick answer first, then the long answer.

Quick answer: OnStartups.
Long answer: The StackOverflow software is just superior for navigating discussions. A major weakness of the BoS forum is that it’s hard to navigate and find old or related threads. StackOverflow has tags, questions can be sorted many ways, users can vote on them.
There is also a popularity system, badges and points, so users see it as a game. Best answers float to the top, so you don’t have to read an entire thread to extract a few lines of wisdom.
That being said, the main reason why StackOverflow was such a success for technical quesitons doesn’t carry over to startup “business” discussions. With software, there are clearly good answers. Either your 3 lines of code answer the question, or they don’t.
With startup discussions, it’s much more a matter of opinion.
The ironic part of course, is that StackOverflow was designed by Joel Spolsky. The same Joel who is behind the “Joel on Software” site. I guess Joel is teaching us a lesson: nothing spells owning a space as much as owning its top two leaders.
I wish everyone the same success as Joel.
FairSoftware is a community of entrepreneurs where you can find co-founders, form your first startup instantly and help each other by exchanging advice on how to start web and iPhone apps.
The real reason for the decline of Joel's BoS forum is simple: He started requiring people to log in. You could no longer easily post anonymous questions and read anonymous answers, which is exactly what people who have questions about business, but don't want to admit they don't know everything, want.