I just read an article on the New York Times website that the Justice Department is beginning an antitrust investigation into the hiring practices of some of the best-known companies in Silicon Valley, including Google, Apple, Yahoo, Genentech, and even spreading out to Microsoft.
In the article they refer to anything from a "gentleman's agreement" regarding hiring, where partners informally agreed not to recruit from each other, to some companies having a "hands off" list they would provide to recruiters. I'm sure "recruiting" is different from "hiring", best the general idea was probably not to actively recruit from "partners".
In the consulting industry we didn't have a hands-off list, but we did have written agreements with our customers saying, "We won't hire from you, and you won't hire from us." In the consulting world, you don't want your customers to even think about hiring your employees, and you don't want to get into a situation where their employees want to work for you, and this formal agreement took care of all those hiring issues right away.