There are many technical challenges to bringing back species that went extinct, particularly if they went extinct millions of years ago. Hydrated and warm DNA tends to decay in a few thousand years, though paradoxically they have found bits of DNA and other soft tissue in some fossils, especially in amber (which protects DNA from water). I saw a program on the History channel about a week ago that stated that it may eventually be possible to recreate "dinosaurs" by taking fragments of DNA from ancient soft tissue, and fill the gaps with the DNA of the nearest living relative of the creature (e.g. chicken used to finish the T-rex). Of course, in that scenario, the result would be a new species, even if it phenotypically looks like a t-rex. Another problem that I seldom hear about is that creating a few individuals of a species would not allow enough genetic diversity for sexual reproduction to be viable for future generations (for mammals it requires about 1000 genetically distinct individuals, if I recall correctly). But this, like most limitations, could probably be overcome with more technology.

Technical issues aside, I was wondering when or if you think it would be appropriate to bring back species or create new species. There are many possible scenarios for this:

1. Suppose mammoth meat is really tasty, and that raising mammoths turns out to be the most efficient way to get meat, which wouldn't surprise me as mammoths lived at a time when food was sparse and were hunted by our distant ancestors. Why not resurrect species, or create new species, as a new food source?

2. Some species have been destroyed by human activity alone. Perhaps we could resurrect them and eventually reintroduce them into the wild.

3. Who wouldn't want to see a dinosaur? Now that'd be a theme park I would go to, provided the park rangers have automatic weapons and rocket launchers. (:

4. Some people say that diversity should be celebrated. Well we're the only member of our genus still alive. Is there any reason we should, or any reason we should not, attempt to bring back Homo floresiensis or neanderthals?