A professor in atmospheric sciences at the U of Utah has a great blog and has been writing a lot about the inversions lately: http://wasatchweatherweenies.blogspot.com/ I live in SLC. The air is indeed pretty ugly right now. On the other hand, my house is a few hundred feet higher than the valley floor and is mostly above the inversion.
When we're in a high pressure system (i.e. nice weather) and solar heating is limited due to low sun angle and snow on the ground, this situation is stable. It happens in many valleys in the western US. This becomes a problem in a few specific valleys where a lot of people live (and drive cars). Usually we get a few weeks of this every winter.