I think this Betabeat article is really relevant since it shows that this is happening at the startup level in NYC as well:

http://www.betabeat.com/2011/11/17/poaching-etiquette-how-to...

This is important because it means that new hires are probably getting less equity than they should be getting. Equity and belief in what the company is building should be enough to not require anti-poaching agreements.