If you've been exploring your capital city, you may have noticed something new. Grandmaster's Training Dummies now hang out in the cities, casually waiting for enterprising player characters to come and beat them. (I don't judge their lifestyles, I'm just reporting the facts.) If you're looking at the picture on the right, you're correct in your keen observation -- the clever training dummies even carry the flag of your opposing faction.
The dummies are level 80, obviously in preparation for Wrath's release on November 13th. We were fortunate enough to get them in the content patch, but it does lead to a few questions. What are these masochistic constructions for? What is their purpose? Do they give you anything?
Dr. Boom in Netherstorm was a staple of Hunters', Warlocks' and Mages' theorycrafting. The analysts of the communities would park themselves on a crate, and fire away at the goblin. Using AddOns likes Recount, or combat log parsers like WWS, they'd leverage the information to draw conclusions about which damage rotations and talents yielded the best results.
Target Dummies fulfill the same role, but are a little more universally accessible. (Melee had problems with Dr. Boom, since they could find themselves getting blown up by adds.) They also have benefit of not being out in the field. While safe and protected in your capital city, you can fire away until you feel confident in the data you're seeing.
There's a caveat to the dummies, however. Some spells (like Siphon Life) don't seem to give accurate results. Also, since the dummies aren't the specific bosses you'll be fighting, you're not going to get the exact same numbers as you would against the real thing. They're intended, after all, characters to practice on, not simulate the actual fights.
Edit: As some commenters have pointed out, there are level 70 and 60 dummies to be had also, in the same general area as the level 80 versions.