I'd like the Physics-Astronomy-Math division of SLA to decide on a #hashtag we will use when twittering at the SLA conference. SLA is recommending using the #sla2009 hashtag. Tweets using #sla2009 are retweeted to http://twitter.com/sla2009. This Mashable article explains how to use hashtags.
This website (hashtags.org) does a good job of keeping track of trends in hashtag use.