As luck would have it, I was actually very very peripherally involved in some of the thylacine cloning efforts a few years back.
Don't hold your breath on that count, is all I'm saying. The number of specimens is so small and the quality is such (a bunch of young ones etc, which are the most intact specimens we have) were preserved in a preservation medium which destroys the usefulness of their DNA for anything resembling cloning efforts.
I'd actually argue that if the (staggeringly enormous) technical hitches can be overcome, then there's certainly ecological room for some extinct creatures to be brought back, but ptobably not all. The thylacine would be right up there, and would probably be great to have in tassie right now, as it'd keep the foxes and cats down a bit and would help take up some of the apex predator slack while the devils are struggling with the facial tumour disease.
The passenger pigeon is an interesting one - in its day it was probably the most common air-breathing vertebrate in existence, with flocks of quite literally billions of a time being recorded, and as such it had a huge impact on its ecosystem. People just used to stand under flocks, point shotguns vaguely upwards, and kill dozens at a time without aiming. The catch is that these days we think that they relied heavily on sheer number as a survival trait, a bit like small schooling fish do today - once there was only a few million left their social structures (roosting, breeding, migration) broke down, their breeding slowed, and their numbers plummeted. How do you generate a self-sustaining population from a single-digit population of very expensive and very genetically similar clones, in that light? You'd end up with a couple of very sad cage birds, with no hope of survival in the wild.
Mammoths are a different kettle of fish - I reckon there's probably more chance of us successfully cloning a mammoth than most other extinct critters (assuming we find some good frozen ones in siberia before global warming thaws them all out and makes their dna useless...) but the ecological implications are far bigger. Mammoths and elephants actually create their own ecosystems - they disrupt earth and knock down/uproot trees, and their crap is a really powerful fertilizer in often-marginal soil (read Tim Flannery's The Eternal Frontier for more info - regardless of what you think about his climate change work, the bloke is a brilliant paleontologist). But at the moment, so many of the other critters that evolved to exploit the mammoth-provided ecosystem have gone too, all the sabertooths and the antelopes and the woolly rhinos and even down to the dung beetles and the microbes that populated the mammoths's gut cavity (mammals tend to inherit their mother's gut flora - in my work with roos, the orphans often have problems if treated with antibiotics when mum's not around, cos it kills their digestive microorganisms and they can't get help from mum, and we have to feed them crap from a healthy adult roo to get them going again). I'm simply not sure how it would work. Though if you got enough of them breeding it might help preserve the Siberian tiger for a little longer, providing it with another food source. Then again, what you'd probably succeed in doing is to provide the poachers who are driving the tiger extinct with another lucrative target. Sigh. Humans, eh?