"Private tracker" refers to a BT tracker that requires a membership. Hypothetically, at least, it's harder for some goon at the RIAA just to hop on one of these, slurp all the IP's, and send letters to the ISP's.
Of course, in that case you're still using a tracker, which is an obsolescent way of getting a torrent.
Course, you wanna talk "obsolescent", an encrypted Usenet service will cost you
$6.67 a month or a nickel a gig. I don't think we'll ever hit a point where a typical luser decides that Usenet is the best place to get the latest Michael Bay movie, but expect more filesharing services to start using SSL.
Paid proxies seem like a potential Next Big Thing, too. Think of something that obfuscates your IP as effectively as Tor but costs money and doesn't come with the same massive performance hit.