Twitter is still new to everybody. It's still evolving. Most twitter users don't see it as anything other than another over-sharing tool. A few months ago I would have classified it as a stupid novelty, but I've seen what it can do, particularly for conferences. Peter Anghelides and Paul Cornell made great use of it at Gallifrey. Eastercon attendees this year were very active on twitter.
A twitter account for the organization is a great way to get reminders and updates out to members. That's something Anticipation is doing well.
A defined "hashtag" (like #amazonfail) allows both attendees and staff keep tabs on public discussion (much like a google alert; y'all do have a google alert on "XYZXYZ," don't you?). There's no formal process to create a hashtag, just pick one (preferrably one that's not going to be used for other stuff by other people) and start using it.
At-con I would put up posters publicizing the hashtag. I would also put a workstation in ops and/or security offices with a client that could monitor the account for replies and direct messages and also monitor member posts that have the XYZXYZ hashtag. Maybe program ops too, there is likely to be a fair amount of chatter about how program items are going. Twitter makes for a very effective back-channel without needing extensive internal IT support or even wi-fi in the conference facilities.
It's not something to really depend on (don't even suggest that it's an emergency contact system in any pubs), but it's a great supporting tool.
Go to http://search.twitter.com/ and search on #gally and #eastercon for examples.