This functionality is not supported out of the box and it will take certain amount of code to accomplish it. Have considered using RadDropDownButton and a TextBlock in the data template of the ListBoxItem. I've prepared a small project that demonstrates this functionality. Have a look at it and let me know if this will work for you. If not, contact us as soon as possible so we can further assist you.
On a side note, I'd like to inform you that we've just released an online tool that allows you to reduce the size of your Silverilght applications. For more information, please visit http://blogs.telerik.com/blogs/posts/10-06-10/telerik_assembly_minifier.aspx
All the best,
the Telerik team
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