One idea to swarming: why do you need to find out which files are popular and download them "in the background"? You would cause even more traffic...

The only thing you need is to make a cache, size depending on connection speed. If an ADSL user has a 2G cache file, it should be OK for him/her nowdays... If you download something it goes to the cache, then it will be copied in the download folder. If your download exceeds the 2G limit, the files that where most uploaded from you will stay in the cache, the others will be replaced with the newly downloaded ones - so no freeloaders.

And for the modem user problem: those are not the only ones, who "steal" bandwidth. 56/36k modem user is able to upload at 4k/s, download at 8k/s (approx.).

ADSL/Cable 384/64 or 512/128 - the difference is bigger, so they (me too) are getting even more than they give....

Multicasting? Wouldn't it be able to do?