For planning this year, we relied on Google Docs and Google Spreadsheets as the primary tools. This worked well with a small group and a small set of sessions. Likely the Wiki would be much better for something larger.
There have to be lots of examples like this out there. Can someone help out with pointers to a few?
Update - I was just pointed to Nonprofit Technology - Conference Wiki that pointed me to a couple of different posts that might be interesting such as: Collaborative Models for Capturing and Sharing Conference Notes at Nonprofit Gatherings. Of course, then I also checked eLearning Learning - Conference Wiki and found an old post of mine: Social Conference Tools - Expect Poor Result. These talk about slightly different issues, but still might be interesting.