I recall the local media reporting on the recent swarm of quakes in the Sierras. They were speculating that they were indicative of magma movement, and were talking about the possibility of Mammoth Mtn. blowing like Mt. St. Helens.
-PJ
The Long Valley caldera (of which Mammoth Mtn. is just one edge) explosively blew out about 750,000 years ago. It spread ash all over the western part of the North American continent. The area is geologically active, with minor ash and cinder-cone eruptions, as late as the 1700's.
I did some preliminary work on dating the ash layers for a proposed Masters thesis over 20 years ago. There remains in the area a relatively shallow, active magma chamber (not nearly as shallow as the one at Yellowstone!), which has been related to the earthquake swarms the occur there from time to time.
It is unlikely that there would be a Cascade-type eruption in the Long Valley caldera region. More likely would be a small, but impressive, cinder-cone eruption.