Here is an e-mail from Iain Murray on the topic:
This is an old staple of Velikovsky-type catastrophists (as well as flood literalists) and belongs more in the “pseudoscience” than “junk science” categories. Fewer than 50 of these frozen animals have been found and most were decayed or injured in some way or another. The famous Berezovka mammoth, for instance, was covered in mud which then froze, not ice. It was essentially mummified, as virtually all of these were.
Flash-freezing isn’t needed to preserve vegetative matter like that supposedly found in stomachs, by the way. Skeletons have been found with vegetation located where the stomach would have been.
And if the odd mammoth succumbed to a global flash-freeze, what happened to all the other, more numerous species? There’s a Far Side cartoon with a neanderthal caught in an outhouse, frozen in a glacier. That should have happened at least once…
There’s more on the subject here.”