If you read this web site regulary, you will have seen a poll to survey user wishes.

The last one is this entry. As you can see, XMP metadata support is on the top of the list...

Andreas Huggel (Exiv2 library author) has work recently to support XMP and provided a first alpha version to test. I have patched libkexiv2 and digiKam for KDE4 to support it.

Well now, digiKam and Showfoto are able to display XMP contents on Metadata sidebar !