Even in a state like California, which is very employee friendly, the OP and the company trying to poach him would probably have a difficult time if there's an anti-solicitation clause in place. The OP is still employed by the consulting firm, and actively engaged in work for the client company. California court cases have suggested this type of scenario might not be a good fit for 16600 protections[1].

If the OP now believes he's underpaid and wants to look for greener pastures, he should consider that he doesn't need to join the client company to find them.

[1] http://www.theiplawblog.com/2007/12/articles/trade-secrets/c...

It is rare that an employee is unique enough to be worth the cost of bringing lawyers to the table.