Eh, I don't know about that. If Joe Internet e-mails the booking agent for Frank Ocean and is all "hey man, I'm thinking about Bonnaroo this summer, any chance Frank is there?" and the agent answers, what's the problem? There's no expectation of privacy there. If anything, it'd be inappropriate for the agent to leak it, but I don't see anything wrong with asking.
Also, I think asking artists on Twitter and Facebook directly is fine, right?
It has nothing to do with the expectation of privacy. I think it's unethical to ask in the first place, particularly with the intention of posting it here. Booking agents aren't supposed to discuss the line-up in the first place and I don't think there should be a thread on Inforoo that encourages people, directly or indirectly, into coercing the info out of booking agents.
I don't think it's unethical to ask a simple question. It's up to the agent whether or not to divulge the information - it's not as if people are offering incentives or anything. When someone a few years back actually POSED as a booking agent to try and get information, that was obviously out of line and coercive. But if you make it clear that you're a fan who just wants to know, you're being honest and at that point, it's up to the agent whether they're doing something they shouldn't be.
I honestly don't care that much (I won't be participating in trying to figure this stuff out), but I think as long as people are honest (don't say you work for a venue and you're interested in booking Artist X, etc.) it seems fine.