Lassen, Shasta, and the rest of the Cascades are composite volcanoes. All the big ones (i.e. the ones with snow...) are concidered dormant, not extinct. All are closely monitored by the USGS and various University researchers for tremor activity, gas levels, and bulging. Shasta, by the way, experiences gas release and bulging constantly. Tremor activity would likely be linked to dike intrusion (just hope it's deep). Lassen's last eruption was in the late 1800's if I remember correctly. Any of those peaks could easily go Mt. St. Helens on us with just weeks/months of notice.
The Mammoth region has a long history of local vocanism, as Nick points out. The Long Valley caldera is for all purposes an extinct volcano. The catastrophic eruption that deposited the Bishop Tuff formation was a once-recorded event, although it did spread ash to South Dakota. Yellowstone was a similar situation, and the likelihood of a recurring similar event at either of these locations is judged to be extremely minute (for one thing, we'd need the build-up of thousands of cubic kilometers of magma in one area --- something we might notice --- plus the fact that the time span between such events is hundreds of thousands of years, IIRC). Around the rim of the caldera, however, several small intrusions have created other formations since the caldera eruption (such as Glass Mountain, Mammoth Mountain, the various Domes, and the Inyo Craters). The Mono Crater chain, a horizontal dike leading north from the caldera to the north side of Mono Lake, has had the most recent activity, with eruptions of < 1 cubic km or so in volume occuring every 250-500 years. If I remember correctly, the last one is estimated to be Negit Island at ~300 years ago.
Mammoth Mountain itself is currently experiencing shallow dike intrusion, which seems to be small in scale. Gas leakage, sporadic tremor activity, and deformation have all been recorded there in the last 30 years. Every so often it seems to act up and scare the heck out of the real estate developers.
If you look at the USGS quake map here,
http://quake.usgs.gov/recenteqs/Maps/119-38.htm
you will notice that there is a constant cluster of shallow, small quakes/tremors (I believe that a "quake" is tectonic in origin, whereas a "tremor" is caused by the fluidic movement of a magma body through rock) east of Tom's Place (under Mt. Morrison/Convict Lake area I think). This is what the USGS is paying closer attention to currently. There are no recent vents in that locale, as far as I know, with Mt. Morrison being mostly comprised of the metamorphic "Morrison Roof Pendant" formation (that is the much older rock under which the Sierra batholith intruded), and the various valleys being glacial in origin.
Another volcanic event in the Mono Crater chain, or even Mammoth Mountain, that stays within the same size limits as all the rest in recent history will, under the right wind conditions, spread ash as far as Hawthorne, Bridgeport, or Bishop.
Earthquakes in our area, like the one in Truckee, are tectonic in origin and simply the result of fault slippage. Occasionally we have a big one in Nevada, but most often they stay below 4.0. Notice on the USGS quake map all the major fault lines in the Great Basin. These are not faults like the San Andreas due directly to plate contact, but rather are the result of crustal expansion that formed the Basin & Range in the first place.
Yikes, is that my longest post evAr?
edit: does my spelling need work or what?