I'm a customer of yours. Out of curiosity, what mailing lists do you participate in?
Actually, I'm sure you'll notice that I'm more active here than on any of those lists... this is probably a bad thing for my technical career development. (perhaps a good thing for my career development as a business person, though.)
Edit: just to be clear, I'm not saying you should ask an applicant what lists s/he is active on... by itself that won't give you that much information (well, except maybe that if a SysAdmin is not on a bunch of security announce lists, something is very wrong but expecting them to remember this data without going through their mailbox is pretty unreasonable.) you should figure that out. maybe ask them what usernames they use on technical forms that they'd like to share? (note, as an employer, you have to be careful not to be creepy when you do this. Most of us have a 'professional identity' we don't mind sharing... but you don't want to look like the kind of employer who goes through employee's myspace profiles looking for drunken party pictures.)
But all these mailing lists, they take serious time . For me, they're really the first thing to go.
Lists I can ignore (e.g. everything but the security -announce lists) go in their own folders to be ignored, most of the time. but they are a good substitute for HN, if you want a more technically oriented and less business oriented discussion, and they are essential if you really want to figure out a tough bug or add a new feature, so having a presence on the mailing lists (even if you don't read them every day) is really useful.