Currently this isn't supported by DataBinding.  here's the note in the docs:

"If you are familiar with WPF data binding, WPF supported property path techniques for retrieving indexed items in a collection, and retrieving attached property values. Silverlight currently does not provide this support. A Silverlight property path is a series of steps into an object model and only supports the "dotted" syntax."
 

If you change the data you're using to a Dictionary (probably a better solution anyway) then you can bind to the Name or Value of each item and is likely going to be a more workable solution.  If you are getting this data via a web service, try the advanced settings to change from Array's to a better collection type.