I just ran across this post on Greg Ruby's Gems about how to make an event app work for your event, which is a pretty good rundown. Here are a few more good nuggets on event apps:
Then there's always this free webinar: How to Create a Mobile Meeting Strategy
And a video:
Wow, we've covered this even more than I thought! But we need to, since it looks like most attendees expect at least a bare minimum app these days.