I have an Android app I'm developing against the Campfire chat service's "API". The code here uploads a file through multipart POST:
http://github.com/klondike/android-campfire/blob/master/src/com/github/klondike/java/campfire/Room.java#L175
Everything after the "dos.close()" line is related to checking the response to detect whether the post was successful.
Not everything in there is necessary for every multi-part post; for example, the X-Requested-With header is specific to Campfire, the User-Agent is optional, and the Cookie is because I have to stay logged in. Also, the "OH MY GOD" comment about spacing is probably Campfire-specific.
I've heard that the latest version of the HttpClient library from Apache has more convenient built-in multi-part support, but the last sync Google performed against it to Android didn't include those features, so here I am doing it manually.
Hope that's of some help.