Armored Trolls are treated as companions and have to be hired.
One of the best news I have heard about Dawnguard ! I hope they have some dialogue. And that we'll discover more about them.
Actually, I have always thought that
some trolls were intelligent beings with some social abilities. They have been involved in clanic structures (in Oblivion, http://www.uesp.net/wiki/Oblivion:Troll#Trolls), they have had leaders (A http://www.uesp.net/wiki/OBMobile:The_Troll_King appeared in Oblivion Mobile, and a place called Frostheim, supposed to be located near Dawnstar, was once home to a http://uesp.net/wiki/Dawnstar:Bestiary). They have been shown to be able to use, and possibly even
craft weapons (http://uesp.net/wiki/Dawnstar:Chung. See also the Troll warhammer and the Troll war axe http://uesp.net/wiki/Dawnstar:Weapons. Trolltooth war axe and Troll cestus are also mentioned here). Besides, a troll cestus would be great for those who rely on hand-to-hand, don't you think? Hope they introduce it too...
I hope that some troll weapons will be reintroduced, and that we'll be able to discover more about them. Maybe they have some culture? Even an embryonic culture? Some troll questgiver would be so awesome too. Just imagine being asked to take part in some ceremony where a troll female is transcended and becomes a new Udyrfykte matron (first idea that pops in my mind) !
Anyways, I hope we'll have some fascinating lore and backstories to explain how they came to be allied of the Dawnguard...
Anyways, the armored trolls look amazing to my eyes !
Trolls can speak English. This was seen in Oblivion http://www.uesp.net/wiki/Oblivion:Easter_Eggs They don't speak English well but it's understandable.
This further substantiate my opinion about trolls, although I never found this poor troll. I am so sad for him.

Also, I hope they'll add ability to use Troll teeth to craft weapons, and http://uesp.net/wiki/Dawnstar:Armor.