[XeTeX] footnotes and Didot

[Cyril Niklaus]

Hi all,
I'm trying to typeset a text with Didot and it has footnotes.  
Unfortunately Didot (the one from Mac OS X leopard) does not have  
superior numbers:

Package fontspec Warning: AAT feature  
'VerticalPosition=Superior' (10,1) not av
ailable in font '"Didot Italic/AAT" at 12.0pt' on input line 70.
Package fontspec Warning: AAT feature 'Numbers=Lining' (21,1) not  
available in
font '"Didot Italic/AAT" at 12.0pt' on input line 70.

How can I change the font of just the footnote number? Do I need to  
redefine footnotemark? And how?

Cyril