As of the recent beta build (13189) that went live earlier this evening, city tabards sold by racial factions (e.g., Thunder Bluff, Stormwind, Gnomeregan, Undercity) are now awarding reputation in dungeons. With the obvious exception of those offered by Gilneas and the Bilgewater Cartel, these tabards aren't technically new to the game -- you can get them at the Argent Tournament on the live servers right now if you want -- but the reputation gains are definitely a new perk.
While you can still get the tabards from Argent Tournament vendors on the beta, each major city also has a faction quartermaster (usually close to the city's flight master and introduced in beta build 13066 in late September, I think) selling both the tabard and a set of level 40 cloaks. The tabards are available at friendly for 8 silver (even at the Tournament -- they no longer require Champion's Seals), but you can't get the cloaks without being exalted. Still, grinding reputation for one of your faction's races should be considerably easier in Cataclysm -- all you'll have to do is get to friendly, buy a tabard and run dungeons as you normally would.
This is a great addition to the game. Not only will you have access to cool tabards even at a low level, but it should make getting other races' mounts and faction discounts much easier as well.