According to Phoronix it is supported:
http://www.phoronix.com/scan.php?page=news_item&px=NjUzMw
and
http://www.phoronix.com/scan.php?page=article&item=intel_x4500hd&num=1
Intel has the best track record for open source drivers so it's not too surprising. Too bad the hardware is fairly weak.
All you should need is a recent kernel...say 2.6.26 or above? Oh, that's what Lenny ships with. If it doesn't work out of the box, the best thing to do would be to install a kernel and xorg (and depends) from testing. Look up apt-pinning for details.