Just this morning I read a claim in Arctic Dreams by Barry Lopez (highly recommended!) that polar bears and brown bears can in some cases produce viable offspring.
As for Neanderthals, the genetics suggest that they were separate species. Until the mitochondrial DNA data came in, they always were considered two varieties of homo sapiens. I don't see how comparative anatomy can trump that.
I think this entire issue has been clouded by politically correct scientists not wanting to imply that "modern" Homo sapiens from Eurasia did not directly originate from Africa. In actuality, BOTH Neanderthal man and Homo sapiens originated in Africa, so this is really a non-issue.
If mammologists were viewing the skeletal remains of populations of hogs, deer, or any other taxa of mammals other than hominids, the apparent differences between Homo sapiens sapiens and Homo neanderthalensis/H. s. neanderthalensis would hardly be considered adequate to separate the populations into two distinct species.
Take a Neanderthal, give him a bath and a shave, and put him in a business suit, and he wouldn't stand out in any major city on earth today.