In my opinion tamers are not overpowered in pvp, except on Siege, where bonded pets should count as your Siege blessed item. I've seen plenty of tamers, both skill players with lots of resources at their disposal, and less skilled players with fewer resources at their disposal, try to be a dismount tamer in pvp, or any kind of tamer. I've seen very few that have had any amount of success. Most end up dead with dead pets, fairly quickly, and realize it isn't as easy as they thought it was going to be, give up and go back to their dexxer/mage, because they have more success that way.
The real problem with tamers in pvp is that most "real" pvp'rs just don't like them, in part because a skilled tamer will force you to change your tactics, and in part for the same reason many people dislike anything, they've decided they aren't cool and they simply don't like them (in other words for no real reason). Most pvp'rs won't be happy until tamers are completely out of pvp, and if they succeed in that quest, I'll be ending my subscriptions to UO and moving on.
As for whether these new dragons will be over powered in pvp. I'm not so sure they will be any more deadly than a beetle or dragon combination with a bake or a mare. They will be nasty, no doubt, but I don't think they will be any nastier than those combinations, and you can't be mounted and use them at all.
And there are changes coming down the pipe that will likely make all pets less effective in pvp than they already are. By the time SA is released, it will be possible for just about any veteran player to put together an all 70's suit with all the mods they could wish for. This means that players are going to become, on average, much stronger, relative to mobs in general, including pets, which will make pets less effective in pvp. Unless proficiencies are back on the table, in which case what I just said will not be true...
edit: And both Nico and I have already tamed at least one dragon each on Test, so they are definitely tamable. As has already probably been said, they are a five slot pet.
edit again: They are like hiryu's, cu sidhe and reptalons in that their stats are halved after taming, but they are still badass without a doubt, even after that.