This study entitled, "How People Really Use the iPhone" provides some great insight into what works and what doesn't work in touch interfaces.
It ends with 8 Rules of Thumb that sound pretty good to me.
1. Take Advantage of Learned Behaviors
2. Avoid Interaction Inconsistencies
3. Provide Clear Conceptual Link Between Widgets
4. Put Space Between Action Widgets
5. Plan for Accidental Overswiping
6. Don't rely exclusively on Multitouch
7. Provide Visual Feedback For Taps
8. Provide Interaction Affordances
Thanks to http://www.touchuserinterface.com for hunting this one down.