Is the act of soliciting a player to pass along information spying fair game or is it a dirty move? What if that player approaches you? Is it wrong to accept?
Players approaches you and gives you info, or asks if you are interested in info? If he/she gives you info and you ignore it, that is just silly as you are hurting your alliance by not telling them whatever it is. At the same time the info could be incorrect, and this person can be coming to you with the intention of throwing your alliance off and weakening them by always making them prepare for the opposite of what is going to happen. He/she could be helpful, but then that brings about the question of why he/she wants to give you info. The player might benefit from the relationship a lot more than you do, and you have to question that players loyalty if he/she walks up other alliances, knocks on the door, and gives info without being asked.
Soliciting a player, well when I used to lead a few years ago I used to do that. Would have conversations with other leaders, and would get information that they intentionally or unintentionally said. I think it is fair game, I mean you can't make them talk. To plant spies in other alliances though I believe is unnecessary and not right. Especially now in days where even pacts have spies in each other, that is a bit overboard imo :\