For a period of time after patch 4.0.3a launched, Wrath of the Lich King dungeons did not provide reputation gains if a new faction tabard was worn. This was hotfixed, however, shortly after Sapperwix's post on November 24: http://us.battle.net/wow/en/blog/1232869#blog
After speaking with our designers further (as promised earlier in this thread) and reexamining how our hotfix was applied together, Inushi was indeed correct and new faction tabards will only provide reputation gains for the championed city in Northrend if the dungeon is Heroic or otherwise level 80. This is how the current tabard system works for Wrath of the Lich King, but we may consider revising the system to include all dungeons, regardless of level, in a future patch.
Apologies for the confusion! I've already amended my first post to ensure it reflects this information.