PlayerId[number here] Are usually people who haven't logged in yet. You can try it yourself. Just log out in-game (By simply typing in a wrong password), and connect to a server, you'll show up as "playerID[number here]".
Although, there are a couple of actual usernames out there that are named playerID[number], so they might be who responded to your whispers.
EDIT : Damn it box :< But I never knew that they could answer whispers, box might be right on this one.
Last edited by EJM; Jul 20, 2012 at 08:02 PM.