Via Honor's Code and Pwnwear.com, comes an observation at Blue Murder that, in Cataclysm, a 10-man tank will have just as much health as a 25-man tank. Therefore the 25-man boss cannot do more dps to the tank, though, because the tank will not have more stamina than in 10-man. However, 10-mans only have 3 healers, while 25-mans have 6-8 healers, so what are all these extra healers supposed to do?
I think this observation is looking at Cataclysm through the lens of the current game.
Right now, the most important variable for a tank is her Worst-case Time to Live. Because bosses hit so hard and fast, a tank needs enough effective health so that she can survive long enough for a heal to land. This is why Effective Health (Stamina + Armor) has been the king tank stat for so long. Effective Health directly increases your Worst-case Time to Live. Increasing your Time to Live from 2.0s to 3.0s makes a huge difference to the healers' ability to keep the tank up.
But Cataclysm is set to change this, by increasing a tank's normal Time to Live significantly. 2.0s to 3.0s is huge, but is going from 10s to 15s as important? Once the Time to Live is high enough, increasing it further doesn't really help anymore. If a tank has 60k health, but the most damage the boss does to her is 50k in 10s, she has significantly more health than she really needs.
In Cataclysm, the limiting resource is supposed to be healer mana. If a 10-man has 3 healers with 30k mana, then--ignoring mana regeneration--the raid can heal 90k mana worth of damage. A 25-man with 7 healers can heal 210k mana worth of damage. The extra damage can come to the tanks, or go to the raid or whatever. The incoming damage-per-second does not matter as much as the total damage taken and the efficiency of the healers.
Since the normal Time to Live will be high enough such that healers will have time to heal efficiently, tanks won't feel the need to maximize Effective Health. I predict that tank theorycraft in Cataclysm will shift to minimizing Damage Taken, focusing on Armor and Avoidance, rather than raw Stamina. Damage that is prevented in some fashion does not have to be healed, thus preserving the healer's mana. This increases the limiting resource available, and makes it less likely that the raid will wipe because the healers ran out of mana.
Of course, this prediction relies on the design direction where a tank can actually take several hits before dying. If that doesn't come true--if the Time to Live drops to a low value--then Stamina will be king once again.