What on earth is an interested member of the general public supposed to think? I do wish those scientist types would make up their minds. Compare and contrast:
BBC (28-Mar-2007): Mammal rise 'not linked' to dinos
The extinction of the dinosaurs 65 million years ago had little effect on the evolution of mammals, according to a study in the journal Nature…
[A new mammal supertree construction] shows that the placental mammals had already split into [their main] sub-groups by 93 million years ago, long before the space impact and at a time when dinosaurs still ruled the planet.
Reuters (20-Jun-2007): Mammals burst on the scene after dinosaurs' exit
… We wanted to test whether there were any Cretaceous placentals," [John] Wible [of the Carnegie Museum of Natural History in Pittsburgh, whose research appears in the journal Nature] said in a telephone interview. "If the molecular dates are correct, we should be finding things that look like modern placentals in this time period and we are not."
They found that none of these Cretaceous forms of early mammals are related to any living placental mammals. "They are just extinct dead ends," he said.
Wible said his work reinforced the idea that the death of the dinosaurs created an opportunity for explosive growth of modern mammals.
"You've got all of these ecological niches that were occupied by the dinosaurs. They go extinct, and you've got wide open spaces. It's like the Oklahoma land rush," he said.
All joking aside, though, this is a fascinating subject. And, as far as I understand these two studies from the press reports (rather than the original papers), they don't necessarily contradict each other.
… But I could be wrong!
Postscript: It turned out I was wrong: the findings of the two studies are in conflict. More here.