They will need to make the ability to seed/share optional as otherwise they'll face problems with customers with low monthly caps.
For example, 3G/4G based providers and even many DSL ISPS have data caps of 60GB or less, in which case watching a popular movie could result in one's data usage allowance being quickly eaten up.
Even with unmetered connections, Netflix will need to offer some sort of incentive to encourage people to seed, as I'm sure they'll not get away with just making their player silently seed as a hidden or system tray process like most BitTorrent clients do.
One incentive that may work is if they offer credit for seeding. For example, 1c/GB, so those with fast unmetered connections would effectively get Netflix for free for using their bandwidth.