Although this is all Theory (and will take a while if it comes to pass).
There are a few futurists who predict Mega cities, (guess what, SD is one of them). The Mega cities will extend quite a ways, as long as you can get to the metro rail station, you will be within the mega city.
On the Subject of Temecula, I think whats going on in Temecula is more a function of too many homes sold to Speculators and too many homes sold in 2005 and 2006, really that is where you see the large number of foreclosures, the homes sold in 2005 & 2006, most of the places where homes were sold in 2002 & 2003, do not show this issue.
In the case of Temecula not really a mass exodus from an exurb just a lot of homes sold way over priced in 2005 and 2006 .
On the other hand those with the means will rent out their Temecula homes (no one will sell voluntarily at these levels) and move to say Carlsbad in a hartbeat if it becomes resonable to do so, but no I would not call that a mass exodus of exurbia .
