If you have read the UI Guidelines for Windows Phone 7 (If you haven’t, get it now) you notice that in the discussion of the ApplicationBar it states that a maximum of four icons can be added. I noticed inadvertently that if you add five icons in your XAML your app will compile and run but only the first four are shown. If you add a sixth icon the designer will present you with an error:-
It appears that this is a bug in the current CTP release and the control should support only adding four items. It previously supported five but this was reduced based on user feedback, obviously the validation in the current CTP is not strict enough but I expect this will be fixed in the next release.