That depends on how you describe a species. The problem is that the recreated mammoth would probably made from modified elephant DNA, therefore having most of the time the gene order of elephants (while having mammoth gene sequences). It would therefore almost certainly be impossible to cross them with the original mammoth. Nevertheless, aside from chromosome arcituctual influences that we do not yet understand would you have a mammoth that is phenotypically identical to the original (and not be a cross between elephant and mammoth). If there should be a successfull genome writing program, it would be possible to recreate the original genome and you would get the original species. The T-Rex clone is different as we do not know (and almost certainly will never know) its genome.

>We're engineering new species, not resurrecting extinct ones.

I think this is way more honest.