On January 18, I posted about the ongoing swarm of earthquakes at Yellowstone National Park. It's still ongoing and, over the last two weeks, there have been over 1,500 small earthquakes at essentially one location in Yellowstone! Here's a real-time updated list from the United States Geological Survey (USGS). Here's a map of Yellowstone, located in northwestern Wyoming, showing where the earthquakes are occurring.
Earthquake swarms do occur at Yellowstone. Last year, in December 2008 to January 2009, there were 900 earthquakes and back in 1985 there were 3,000 in a particularly large swarm. All of these current eartthquakes appear to be located about 10-11 km in depth and aren't shallowing with time (as you might expect if they were caused by magma movement - something we don't want to see).
Anyway, seismologists at the USGS still believe these earthquakes are tectonic and not magmatic. They're just stress readjustments along faults on the edge of caldera and no big deal. Hopefully they're correct.