The North American grizzly was first described in 1815, and Lewis and Clark were the first scientists to collect specimens. Lewis described the first specimen as "a most tremendious looking anamal, and extreemly hard to kill."
C. Hart Merriam is one of the fathers of modern mammalogy and a truly excellent scientist. He was a founding member of the National Geographic Society, the American Society of Mammalogists, and an early head of what would become the US Fish and Wildlife Service.
Between 1898 and 1929, Merriam described about 70 new species or subspecies of what is now classified as Ursus arctos, and more than 90 subspecies or species were recognized at one point. Subsequent genetic work and more specimens revealed that many of those taxa were actually just natural variation in color or geographic size variation which didn't reflect real separate populations. This is a valuable lesson and a common pattern. Many of the new species that are being described each year are found in museum drawers, and some existing species are folded into the same name by the same process.
The hybridization of a polar bear (Ursus maritimus) with a grizzly bear (Ursus arctos) itself might not be enough to indicate that the two are the same species. Such hybrids aren't news, reports of such hybrids date to before 1972 when Gray's Mammalian hybrids: A check-list with bibliography was published.
What does make people suspect that the two species are not distinct is that an extensive genetic analysis, using the full mitochondrial DNA and two nuclear genes, shows that the polar bears are more closely related to some grizzly bears than other grizzly bears are to the same populations. This suggests that the polar bear may not be a distinct species, or that the major distinct clades within the grizzlies are actually different species, and given the similarities between those groups, and their ability to interbreed, the former option is more likely.
Most people encountered Mayr's "biological species concept" in introductory biology classes. But the concept more preferred by many systematists (at least here at KU) is the evolutionary species concept. In this formulation, rather than requiring reproductive isolation (Mayr's criterion), two taxa are different species if they are different evolutionary trajectories. Genetic isolation and differences in selective pressures both factor into that analysis. Reproductive isolation is necessary to maintain two species, but that isolation can be geographic, behavioral biochemical or genetic.
By that standard, calling the polar bear a new species is certainly reasonable. New contacts between what were isolated populations may be ending this incipient speciation. Those new contacts are being driven by climate change, which forces grizzlies further north and pulls polar bears back from the unstable ice further north. More contact will mean less isolation, and the unique characteristics of the polar bear may be lost.