With Tweet Bubbles you can get floating notifications from a hashtag or user:
TweetBubbles is a tool which can make your presentation more exciting by live feeding Twitter during events or seminars.
Keynotetweet lets you post something when a slide comes up.
Simply add text inside the tags [twitter] and [/twitter] in the presenter notes section of a slide and when that slide comes up in the presentation the script will grab that text and send it to Twitter on your behalf.
I can see a very cool presentation being done by using these tools.