Have you thought about using one of those automated voice messaging services to promote your event? I've been on the receiving end of one lately (not for ASAE and The Center's Great Idea Conference, but reading this post on the topic on Acronym by Ann Oliveri made me think about it). I know some people love them, and from press releases I've gotten, the companies who provide them say they improve attendance. But I'm with Ann: They make my skin crawl.
Is it worth it to use these services if, say, 75 percent of your audience loves them, but they turn off 25 percent to the point where they feel tele-spammed and those warm fuzzy feelings turn sharp and spikey? I wouldn't do it, but then again, I'm in that 25 (or whatever) percent.