During a Windows Phone 7 course a few weeks ago I was asked about limitations in the ApplicationBar for Windows Phone 7 applications. I mentioned my thoughts on the overall design of the ApplicationBar – e.g. the missing possibilities of sharing an ApplicationBar between pages (there are some workarounds!).
The student asked about any limitations in the number of items – to be honest I did not know the precise answer. I quickly discovered that the limitation is 4 items. If you try to add more items then 4 items you will get a warning in design time and XamlParseException at runtime (inner exceptions is “Too many items in list”).
thomas martinsen 
Another limitation, not the number of items, is binding. If you need language specific interface you cannot bind labels to the text property of the menu item.