I've heard a claim that a torrent with zero seeders can actually be downloaded in a finite amount of time, because not all people subscribe to the same torrent file, but you can still find them using other techniques such as DHT. Is this true in practice?
From this BiTorrent FAQ:
When there are zero seeds for a given torrent (and not enough peers to have a distributed copy), then eventually all the peers will get stuck with an incomplete file, if no one in the swarm has the missing pieces.
When this happens, someone with a complete file (a seed) must connect to the swarm so that those missing pieces can be transferred. This is called reseeding. Usually a request for a reseed comes with an implicit promise that the requester will leave his or her client open for some time period after finishing (to add longevity to the torrent) in return for the kind soul reseeding the file.
DHT clients do not need tracker to operate, so it can download without any seeders, as seeders are reported by tracker itself. Good explanation of DHT relating to torrenting can be found at : http://www.tamiltorrents.net/forums/showthread.php?t=8819