Mostly, orchestras have used performance tweeting to communicate with people who aren't in attendance.
The Edmunton Orchestra hired this guy to live-tweet during a performance in April. His link to his tweets didn't work, but based on the blog post I linked here, my impression was that he had a very "man on the street" approach. I'm guessing that in the long run, most people would want something slightly more informed.
In November, the Modesto Symphony allowed (or hired? I don't know the arrangement) Modesto Bee arts writer Lisa Milligan to live-tweet during a dress rehearsal. I'm sure this is happening elsewhere -- the Modesto thing happened to turn up high enough in an online search that I found it.
Item #3 in this post from the League of American Orchestras describes several social-media tactics that performing arts groups in Columbus have tried/are trying. Opera Columbus had a Tweet-up: live tweeting during a dress rehearsal for Opera Columbus. The difference here was that several tweeters were doing this at once, providing different perspectives on the music (or the general experience). Again, they had to sit in particular reserved section.
This post by an independent arts promoter and artists' rep delivers thoughtful analysis of her experience tweeting during a performance. Keep reading, and you'll read what David Lang and Hilary Hahn have to say about it, too.
Finally, I offer a caveat on my own suggestion on audience participation through Twitter: this is definitely a limited-time idea. It will get old fast, and any orchestra that tries it will have to be very alert to how it's working. Maybe Richmond isn't ready for it yet; maybe another city is. Maybe Richmonders will like it for half a season, maybe two seasons; maybe another city will like it 2 or 3 times a season for two seasons. Tweeting may be a fad; using online media is not. In any case, it's too easy for orchestras to fall into habits of thinking that say: Because we play music that's timeless, we can't associate with fads.
Here's the final bit of Midgette's Post article:
What classical music audiences and administrators too often forget is that all these new technologies are mediums, not messages: How well they work depends entirely on how intelligently they're used in the service of what they're trying to communicate. The real point is that the message -- the quality of the music, not some watered-down version of it -- deserves to be disseminated widely, by any means possible.