Yellowstone National Park has been trembling with over 200 earthquakes over the past few days (today's January 18). Here's a list and map of some of them. Nothing too large so far and nothing to worry too much about yet.
Last year, in December and January of 2008/2009, there was a swarm of over 800 quakes under the north end of Yellowstone Lake with no large earthquakes. The recent swarm is over on the west side of the Yellowstone caldera near the Montana/Idaho border. It's also close to where the largest swarm ever recorded occurred in 1985.
Yellowstone does have a potential for very large earthquakes. On August 17, 1959, a magnitude 7.3 occurred during the height of summer tourism season and killed 28 people. Most probably also know (courtesy of TV Supervolcano hype) that Yellowstone has had inconceivably large volcanic eruptions in the past as well!
Odds are, this swarm will settle down in a few weeks. You never know, though, and a large earthquake would not surprise geologists at all.