Blizzard has announced plans to "bribe" (as Tobold points out, their words) players to tank 5-man heroic dungeons in an effort to address "unacceptable queue times currently being experienced by those that queue for the DPS role at max level". The bribe will consist of gold, stuff that can be bought or sold for gold, and a chance at rare cosmetic pets or mounts. This is like giving an aspirin to someone who just walked in with a gunshot wound; I don't know that it will cause significant harm to the game, but I also doubt that it will solve the problem.
What It's Not: Gear
Blizzard chose not to include gear on the list of rewards, and I agree with their reasoning. Tanks (and, to a lesser extent, healers), already get groups faster and thereby obtain all of the existing loot faster than DPS can due to shorter/instant queue times. Paying out more "valor" currency per dungeon run will help existing tanks finish getting all the gear they want and then stop running dungeons entirely at a faster rate, but my guess is that people who are both interested and capable of switching to tanking for faster currency gain have already done so.
Perhaps adding raid-quality DPS loot (tank loot is not an incentive for people who don't want to tank) might convince more DPS to make the jump, but that way lies the item inflation that trivialized the group content of the last expansion. More to the point, as I've argued in the past, offering this type of incentive does not magically convince players that they like an activity that they previously disliked. Instead, this is precisely what leads players down the road of viewing gameplay as something to be gotten over with as quickly as possible, with all the negative community effects that brings.
What It Is: Optional
So, instead of gear, Blizzard is offering minipets and non-raid dungeon mounts (of which there are about five in the game). I don't have a problem with this, but I suspect that many players who actually care about this sort of thing already have large collections of each. In order to increase the tank pool, you would need significant numbers of DPS who are capable of tanking and enjoy having cosmetic stuff but don't enjoy obtaining them through the currently available methods. I'm sure the number is non-zero, will it be enough to reverse a trend that has seen queue times triple or quadruple since Cataclysm?
This problem begins and ends with the fact that modern holy trinity based MMO's require players to tank and heal in proportions that do not reflect player willingness to assume these roles. I have yet to see any evidence these "bribes" do anything to resolve this problem.