When working with XAML and databinding there is not really much help available to aiding the developer/designer when a databinding is not working. Luckily in .NET 3.5, this changed and we can now include an extra diagnostics trace in a binding, which will show some helpful trace messages about a binding.
One my WPF Disciple buddies Bea Costa published a blog entry about how to do this. You can read more over at her excellent blog entry http://www.beacosta.com/blog/?p=52. I had originally included a link to Corrados (another WPF Disciple, but Corrados link broke, so I had to change to Bea’s post instead).