A paper in this month's Current Biology uses ancient DNA to trace the taxonomy of North American cats, the sabre-toothed tiger and a cheetah-like cat. The DNA was amplified from specimens found in Patagonia (the methods are not online yet). The short recovered sequences show that the sabre-tooth tigers belong in a separate clade from surviving big cats; and the cheetah-like Miracinonyx is a relative of the New World puma.
A tangentially related piece at Discovery Magazine talks about what those pearly sabres were chomping. A big set of fossil teeth from sabre-tooth tigers and wolves, found at a fossil bed in South Carolina, were analyzed for carbon-13 content Carbon-13 is enriched relative to Carbon-12 as you go up the food chain, but this paper makes the additional assertion that C-13 is also enriched in open grassland compared to forest. The C-13 for sabre tooth tigers was much lower than for wolves, though both were top predators. The authors interpret conclude that sabre tooth cats preyed mainly on forest animals.
This reliance on forest may have been the downfall of the cats when the climate shifted.
Finally, (are we to Kevin Bacon yet?) a paper to come out in PNAS uses fossil evidence from Cuba and Hispaniola to suggest that North American megafauna extinctions at the end of the Quatenary period are more closely timed with the arrival of humans than with the climate changes of the ice age. Giant sloths persisted out in the islands until about 4500 BC, well after the last interglacial-glacial transition (between 15,000 and 9,000 years ago). Instead, the closely spaced extinctions in North and South America, and the much delayed island extinctions, are more consistent with humans being the change agent.