Ok, I see how that works. However, that would not be my expected behavior based on how the "Append" mode works and the description of the Append mode from the documentation.
mode after one or more matches are found the DropDown portion
of the control is hidden and the value in the TextBox is appended with the first
It doesn't say you need to use the keyboard and cursor down (which isn't really an option for Append mode since there is no drop-down). It should pop up with the first matched item without having to enter into the drop-down list. In fact, that is how the AutoCompleteBox from the SDK works, which I assume, you are trying to create a replacement for (and I am hoping to replace in my apps with the RadAutoCompleteBox!).
This behavior is described under "Automatic text completion" on this page http://www.silverlightshow.net/items/Using-the-AutoCompleteBox.aspx
So, I am wondering if this was overlooked, is a bug or not the way you want your version of the control to work (which would really be too bad, IMO, since this is a cool feature of the SDK AutoCompleteBox).