You should read this post about creating Stack Exchange clones.
Basically it says:
- Look and feel is not copyright
is copyright (so you can't make
copies of their publicly available
.css and .js files)
- Names and logos
could be trademarks
There's also a whole bunch of questions linked to that question on the same page which ask the same question.
Here is a question which lists all of the clones that have already been built. So basically you are not the first person to want to or who has already cloned Stack Exchange.
On another note I wish I had a dollar for everyone who had a "great business idea" based upon a Q&A site since this site became successful. As many people have pointed out on other sites, the success of Stack Overflow and other Stack Exchange sites is mainly about the community and only partly about the functionality.
If I had a Q & A site I would be more focused on how I could build a community and less about what my site looked like. These guys (Joel and Jeff) had tens of thousands of loyal readers that they were able to bring across to SE when it first started (myself one of them).