Absolutely. Several of my colleagues and friends have run into non-compete issues moving from big companies to other big companies.

One high profile case was Mark Papermaster's move from IBM to Apple: http://fortune.com/2009/01/27/ibm-settles-papermaster-to-joi...

I've seen attempts made to enforce it. We call it the nuclear option because for small companies, we are talking mutually assured destruction do to lawyer fees -- and virtually guarantees the former employee will go bankrupt. And at least in my town, word gets around that you are hostile to your employees, meaning hiring grinds to a immediate halt. If you are a consulting company...good luck with the next contract. No one will want to work for you or with you, and multiple current employees will quit on principle.

At least that is what I saw happen at a former company I was with. All of this was in "Right to Work" state that invalidates non-competes anyway.

Even when they're not enforced and not expected to be enforced, they can still cause problems for someone at a big company trying to be hired by a small company. At a former small employer, we basically wouldn't touch anyone with a non-compete if there was the slightest risk.

It happens in trading fairly regularly.

Yep. Even in states like Illinois where NCAs are non-enforceable, they can still complicate hiring decisions. I've heard of a few cases where companies (particularly in trading) will choose not to hire someone with a NCA because of the risk/time/effort involved in dealing with it.

NCA's used to be largely nonenforceable in IL. The advice I've received is that was changed by court precedent in recent years.