In my previous post I wrote this--but then removed it:

"Note that the Office XP PIAs will work against either Office XP or Office 2003.  This is because the PIAs are backed by the COM object models of Office that are versioned in such a way that the OM of Office XP is a proper subset of the OM of Office 2003.  In other words, if you use the Office XP PIAs, you get the benefit that your code will work against Office XP and Office 2003, but you don't get to use any of the cool new developer features of Office 2003 (like ListObject, XML Support, etc.)."

Although this should be true in theory, it turns out that in practice there are some areas where the XP PIAs will blow up when you use them against Office 2003.  One example I've been told is that if you are an add-in and you call Microsoft.Office.Interop.Word.Application.Open() using the XP PIAs with Office 2003, you can crash Word.

So the current recommendation from Microsoft is not to use Office XP PIAs with Office 2003.