but wait, Microsoft is in WA, so you sign the contract in WA. Can CA really say a contract signed in WA is void?
So in that case anyone in the country can work at any job in CA they want regardless of what their non-compete says?
If you work for Microsoft, in Washington, then there's likely to be a tight non-compete clause. Washington courts are very permissive toward employers regarding non-compete provisions. I can't speak to the specifics, although there was a high-profile case a year ago involving a Microsoft executive who was sued and ultimately was prohibited from working for Google.
Finally, many companies don't enforce non-compete agreements strictly unless you do something wrong, i.e. stealing information from your employer to help someone else directly compete against them.
(IANAL, and I don't even play one on TV.)
(I just did some research about this a few weeks ago. Had Google filed a case first, asking for a declaratory judgment, they likely could have gotten it handled in California courts, and the decision would have gone the other way. Seems like a rather screwy system...)