are promises to work exclusively for employer enforceable in California

Any judge will tell you to keep your promises. However, noncompete
clauses or agreements are not enforceable in California.

Then there is that issue of keeping your word and the positive impact
that has on all your professional relationships with customers,
suppliers, co-workers and in the community. One point of view is to
keep your word because it is the right thing to do, not because it is
enforceable in a court of law.

Sometimes people are required by employers to sign statements about
not competing.  Do you really want or have to do business with them?

fcnsp is correct, although there are two exceptions: 
1.  When a partnership is involved (the non-compete agreement will
apply to a partner in a partnership) and
2.  When someone is selling their ownership interest in a company.  

See http://www.andersenalumni.net/%5CCalifornia%20Non-Compete%20Agreements.pdf