One point in favour of webforums: I'm a freelancer, and whether I'm allowed to attach my (stolen last night by some scabby little scag-head burglar and hopefully soon to be replaced) laptop to a customers network varies, but generally I can't.

Using a mailing list requires either subscribing and unsuscribing a temporary mail account each time I move on to a new customer, or polling the (usually very user-unfriendly) web archives of the mailing lists. A webforum on the other hand, provides continuity, I can see the same messages on site on monday as I see at the weekend at home.

My feeling is that the ideal would be a combination of the two - a webforum with a decent UI, with the ability to subscribe & submit messages by mail, so users can use whichever interface they prefer.