As far as I know, those cases are pretty hard to win (in my non-lawyerly observations of the law). It also varies from state to state, I believe.
I've had to sign a couple of documents that said that any work I created, at work or outside, that was in the domain of the company was owned by the company. This company happens to do software engineering, but I'm not particularly scared since my job has absolutely nothing to do with software engineering. I do, however, do "software engineering" outside of the company on my own time. I believe in order to prove that they own the IP to whatever else I produce, they'd have to prove that either I created it on their time, or that I created it using skills or tools I gained while employed there. They'd also get the IP under the work for hire law, if, say, I happened to be hired to produce some piece of IP, but I'd say that's pretty obvious.
I've had to sign a couple of documents that said that any work I created, at work or outside, that was in the domain of the company was owned by the company. This company happens to do software engineering, but I'm not particularly scared since my job has absolutely nothing to do with software engineering. I do, however, do "software engineering" outside of the company on my own time. I believe in order to prove that they own the IP to whatever else I produce, they'd have to prove that either I created it on their time, or that I created it using skills or tools I gained while employed there. They'd also get the IP under the work for hire law, if, say, I happened to be hired to produce some piece of IP, but I'd say that's pretty obvious.