Due to low lvl cloaks as rewards for city rep, it should work in all old world dungeons. Or as the blue post on the front page says:
City Tabards do not give reputations
This is a bug we will be hotfixing in the very near future. The city tabards should currently work in old Azeroth dungeons (as well as Cataclysm 80-85 dungeons when the expansion is released). The hotfix will make it so these tabards work for Heroic and normal Northrend dungeons, however, there are no plans as of yet to apply this change to Outland dungeons. (Source) (Source)
not quite sure how to get it in a neat blue frame yet but meh, guess the info is there