I've added support for interlaced PNG's. The patch attached is against the subversion repository, tag pil-1.1.7a1-20090317.
I've tested it for all pairs of normal and interlaced files in this png suite http://www.schaik.com/pngsuite/ and some random images. It works OK.
I wasn't sure where to post this, sorry if it's in the wrong place...
Edit: forgot to say, it's read only support.