Tamar Weinberg has an excellent post on the impact of Twitter on presentations. Weinberg shares my view that Twittering in the audience is a positive development as it amplifies the impact of of a presentation. Before you only had a chance to present to whoever showed up; but now you have the possibility to reach hundreds, possibly thousands via the Twitter stream.
Of course, presentation preparation will have to be adjusted for Twitter. If you are rehearsing a client for a presentation, it will be necessary to include some method of monitoring audience Twitter feedback.
And more than ever, it will be necessary to impress upon clients that there is no such animal as off the record. A presentation is inherently a public act.