The one I saw demo'd that I couldn't peg as a bot used XML/RSS lookups to be able to talk about current events and not just mindless banter (which can fool, of course, but doesn't hold up to extra-contextual input). The bot asked if anyone had caught such and such a show on tv last night or read such and such in the news earlier.

I considered, somewhat seriously, writing one (a family of them) designed to talk dirty. The end being setting up a website where subscriptions were sold to chat with live nude girls, with somewhat less than live or nude bots described in the "just for entertainment" clause of the user agreement of course. :)