The case of lions/tigers, dogs/wolves, horses/zebras is not problem for evolution. It is what we would expect. Speciation begins when populations quit interbreeding for any reason whatsoever -- geographic barriers, perceived ugliness, whatever. It doesn't mean they are incapable of producing hybrid offspring. They just don't.
Seems like there was a thread a few weeks ago that went pretty deeply into the whole speciation issue. I don't consider hybridization to be a problem for evolution. If anything I think it demonstrates the connectedness of life. Disagreement over taxonomy doesn't mean speciation doesn't happen. There are plenty of debates over whether some creature should be considered one species or another, or whether it's a new species altogether. I consider that to be a strength of science, not a weakness.