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:
8 Steps to a Winning Mobile Meeting App
How-to Guide to Mobile Meeting Apps
Your Mobile Meeting Apps Questions, Answered
App Map: How to Produce a Mobile Meeting App
What Mobile Meeting Apps Can Do: 36 Features
Mobile Technology Strategies for Meetings
App Happy: How to Put Your Corporate Meeting on Attendees' Smartphones
Then there's always this free webinar: How to Create a Mobile Meeting Strategy
And a video:
[youtube]http://www.youtube.com/watch?v=I7nxuUbN8uc&feature=player_embedded[/youtube]
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.