I would just like to say thanks to MacRumors for the heads-up. Sure 'nuf, my Snow Leopard MacBook was on the old version.
How prevalent in the wild are malicious flash pages that take advantage of these vulnerabilities?
They are essentially non-existent, and if they did exist, you wouldn't be vulnerable unless you had reconfigured your browser to use the Flash plugin to read PDFs instead of using Preview.
Sophos, who wrote the inflammatory press release quoted at the top here, have a vested interest in making it sound awful even though there is not a single reported instance of this hitting a user in the wild. Why? They're trying to sell security products to businesses. (Sophos is also known for being much less respectable in this game than, say, Norton--not that Norton is great, but Sophos tends to live in the gutter).