Is it possible to setup several additional domain controllers (ADC) at remote locations that are connected via medium bandwidth DSL (2-10 Mbit) WAN connections for a single domain (intranet.example.com)? And would it be a good idea?

We have five sites and would like to have extremely high availability if any of the site were to lose their Internet connection. However each site is very small, and all are over a fairly small geographical area within the same region, so it would seem strange to have a PDC for each of the sites.

If it were possible to have an ADC for each site, would the clients use the ADC or just use the PDC if it's available to them?