If you have access to the October issue of asp.netPRO magazine check out the ControlFreak column written by Steve Orr.  Back in March I was trying pass data between a User Control and its page.  This article would've been another good resource at the time.

More than just covering passing data between User Controls he covers a laundry list of questions on how to pass values between pages, controls, and other objects.  He reminds us that all controls inherit from the Control class so they can all interact with each other. These are few I took note of:

• Passing data to a user control.
• Pass data back to the page from a user control.
• Pass a value between two user controls.