I won't recommend it. As far as I can understand, Moonkin tanking was possible in TBC but I beleive it was more of an oddity rather than widely used.
However, the armor mechanics changed in WotLK which make Moonkin tanking difficult, if not impossible. In TBC, armor from cloaks, rings, amulets and trinkets were also multiplied by the modifier when druids shifted. That changed that only leather and cloth armor (except cloaks) multiply when shifting. To counteract the change, the Survival of the Fittest talent was changed to add bonus armor as well when changing to bear. It was originally 66% extra but it was changed to 33% extra now.
SotF is mid Feral and the armor bonus is only for Bear and Dire Bear, thus your armor in Moonkin form will never be really enough for it. On my feral spec the difference that SotF gives is over 5K, and that is with the lvl78 LW set. With decent raid gear, the difference will be a lot bigger.
Another thing that changed is that Dire bears do have a type of block now. At lvl40, you get Savage Defense, which gives you a shield equal to 25% of your AP evertime you crit in Dire bear.