Rumors have it that hackers are detecting and cataloging vulnerabilities that they're holding in reserve for the day Microsoft stops support for XP, after which they'll have a field day exploiting known vulnerabilities, secure in the knowledge that the errors will remain in place until the victims finally dump XP.
For example, Vista has a substantially-rewritten networking stack. A networking exploit in Vista would not necessarily translate over to XP.
On the other hand, there's a lot of legacy code around in GDI+ for decoding graphics formats. A file format exploit would be highly likely to carry over to XP.
That makes me curious as to just how much legacy code still exists in Vista/7/8/8.1, and where. I guess it's time for me to do some more research.
...or until someone else patches them; remember the WMF exploit at the end of '05? There was an "unofficial", but just as effective, patch released before Microsoft released theirs.