<p>I want to digest a multipart response in C++ sent back from a PHP script. Anyone know of a very lightweight MIME parser which can do this for me?</p>

I know this may be too little, too late, but I had a similar need.  I wanted a mime parser that just did the encoding and decoding of the MIME.  For the sake of completeness and for Google-ability I thought I should put my findings here.  Keep in mind that I was not interested in send and receiving mail, just encoding and decode MIME.  Here are the libraries I researched for C++ MIME:
http://www.vmime.org/
- Looks like too much. Contains pop, SMTP, tls, IMAP, could remove this.  Posix and windows.
http://codesink.org/mimetic_mime_library.html
- looks promising.  Very nice api for reading in and creating messages. Went with this one. Not too heavy.  Had to “fix” 2 things. First for multipart the code was just check for the string “multipart” and did not recognize “multipart/mixed” and did not write out the parts. Secondly, I had to hack the mimeentity write code.  It was just writing out the std::deque Field and since these are strings it seem to be doing so alphabetically.  This is problem because the MIME-version has to be the first field written out. So I fixed this. I also had to add support for Content-Length.
http://www.mozilla.org/mailnews/arch/libmime-description.html
- Hard time finding this.  Had to download the whole package. mailnews\mime\src
http://www.example-code.com/vcpp/smime.asp
- Didn’t consider because it had no source code and was windows specific.
http://httpd.apache.org/docs/2.2/mod/mod_mime.html
- Couldn’t find this C implementation.
I chose Mimetic for my needs although I had to add a few things to it.  None of the parsers I found handled the optional fields (Content-Length, etc..) very well.  I also needed it to support multiple platforms (Windows, Linux, etc..)