Where I work, we administer/support a LOT of vendored apps, and we see poaching, in both directions ALL the time. You see some guy working for the vendor active on their forums, and six months later, you see him again, only with the logo of some customer on his profile. Or a third-party consulting firm. Then you see it vice-versa, the vendor has poached a power-user from a customer. The better players are "adults" about the whole thing, and see it as a part of the business.