Engineers often misinterpret the provisions of the law.
" (1) Relate at the time of conception or reduction to practice of the invention to the employer's business, or actual or demonstrably anticipated research or development of the employer; or (2) Result from any work performed by the employee for the employer."
#1 is pretty vague, and it has been interpreted broadly by most courts. Almost every engineer i've talked to think it means "unrelated to the work i'm doing for the employer" (which is really part #2 of the law). However, it in fact says that not just "work related to stuff you are doing for your employer", but "work related to stuff your employer does at all or has said they will probably do in the future" can be owned by the employer, even in your spare time.
So if you work for a company that does a lot of things, you should not expect to own anything.
In practice, this is not very different from the laws of most states.
While I am a lawyer, this is not legal advice. Just trying to correct a misconception.
In fact, in Germany we have the "Arbeitnehmererfindungsgesetz (ARBNERFG)" [1] which stipulates that your employer has first right to all patents and inventions related to your work, even if you create them in your spare time. The basic reasoning is that your employer supplies you with all tools and ressources required for the invention in question. It would be way too easy to just clock out, return to your desk and a minute later write down the world formula and sell it for billions. You're entitled to a compensation though.
The law looks a little stupid when applied to computer science but makes much more sense in a research or engineering context.
[1] I just adore german legal terms.
http://www.leginfo.ca.gov/cgi-bin/displaycode?section=lab...
2870 is what you're most interested in. IANAL, this is my lay understanding of the law.