I am in Nidhogg, and Tiandelin that posts on here is in Monolith. There are also a few other noteworthy prot pallies, and the server as a whole is VERY receptive to paladin tanks.
Vicarious has downed Illidan, and some of the noteworthy guilds would be Conclave of Sin, Nidhogg (we just downed Vashj and working on KT, and they actively recruited me as a pally tank), Monolith (though I got a lot of pressure to go holy with them...), Water Sleeps, and Gnomish Tea Party. E Pluribus Unum (EPU) is quite far in progression too, but I really don't know anyone in that guild.
Be aware that any guild with high progression will require you to be similarly geared, though, so you might want to set your sights for some of the more casual guilds until you can work up your gear.
The server as a whole, though a little heavy in population, is outstanding for PVE horde guilds. Very good progression, and a (relatively) high maturity level from my experience. Like Stalkerzero said, though, with high acceptance of pally tanks means you will not be handed a tanking spot without proving yourself... there are a high population of tankadins to fight with.
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<3 Bram
The enemy outnumber us a paltry three to one, good odds for any Paladin!