I'm very sorry for my lack of answers. I work by subscribing to forums and getting news via email, and somehow I forgot to subscribe to this most important of our boards...
Anyway: We don't have filters to hinder uploaders from downloading (they would get an incentive to begin to lie about their shared files if we did).
But: In Gnutella every downloader automatically supports the downlaoded files because he/she is added to the download mesh. So even if someone doesn't actively offer something he/she adds his/her upload bandwidth to the
download mesh of the file.
You can see that when you download some popular files. You'll have new uploads for that file almost at once, because you enter the download mesh (which works just like BitTorrent, but completely decentralized).
Besides: A late but warm welcome to Phex. I wish you happy sharing!