As an ecologist, I can vouch for many of my colleagues accepting the Alvarez theory for the K-T extinction event. There are some doubts, as others have mentioned, that the pattern of extinction may not be consistent with a meteorite impact, but no one can really claim to understand the affect such a dramatic change would have on the biosphere yet. However, my evolutionary bio text has this to offer:
-Skelton, 1993. Evolution, p. 792. (Incidentally, p. 793 also mentions that marsupial mammals suffered more in the K-T than placental mammals, but it does not include marsupials among the groups suffering “major” losses.)
This could be taken to mean that a single event like a meteorite impact cannot explain three extinction events. On the other hand, it may be possible that the dramatic climatic changes brought on by such an impressive impact played an important role in subsequent extinction events.
I don’t think very many ecologists would support the notion that all mass extinctions are caused by meteor impacts, though. I’ve heard of some interesting alternative hypotheses on the Permian extinction, including drasitc changes in the area of continential shelf due to continental drift or a massive anoxic event triggered by changes in oceanic circulation (sorry I don’t have cites on these right now). The extinctions in the Permian seem to have taken place over a very long period of time, and therefore would not be a good candidate for the meteor mechanism.
With reference to the possible periodicity of ‘mass’ extinction events, it may be useful to examine: Raup, D.M. 1991. Extinction: Bad Genes or Bad Luck?