Scientist say that may realistically be able to regenerate the Mammoth. The NY Times reports:

Scientists are talking for the first time about the old idea of resurrecting extinct species as if this staple of science fiction is a realistic possibility, saying that a living mammoth could perhaps be regenerated for as little as $10 million.

I think this would be the hunting equivalent of space tourism. The richest hunters from all over the world would be competing for the title of “First Mammoth Hunter in 10,000 years”. Ancient animal hunting would become an industry in of itself and firearm manufactures would no doubt jump on the opportunity to manufacture the .900 Mammoth Express.

Wooly Mammoths by Mauricio Anton (CC-By-2.5)

Unfortunately a T-Rex will not be able to be bred:

The same technology could be applied to any other extinct species from which one can obtain hair, horn, hooves, fur or feathers, and which went extinct within the last 60,000 years, the effective age limit for DNA.

Scientist seem to be eternal optimists. I don’t expect to see one in my lifetime.