A search through the PIL source (1.1.7) tells me that it can recognize XMP information in Tiff files, but I cannot find any evidence of a documented or undocumented API for working with XMP information using PIL at the application level.
From the CHANGES file included in the source:
+ Support for preserving ICC profiles (by Florian Böch via Tim Hatch).
  Florian writes:
  It's a beta, so still needs some testing, but should allow you to:
  - retain embedded ICC profiles when saving from/to JPEG, PNG, TIFF.
     Existing code doesn't need to be changed.
  - access embedded profiles in JPEG, PNG, PSD, TIFF.
  It also includes patches for TIFF to retain IPTC, Photoshop and XMP
  metadata when saving as TIFF again, read/write TIFF resolution
  information correctly, and to correct inverted CMYK JPEG files.
So the support for XMP is limited to TIFF, and only allows XMP information to be retained when a TIFF image is loaded, possibly changed, and saved.  The application cannot access or create XMP data.