Using WP 3.0, Domain Mapping 0.5.2 and W3 Total Cache 0.9.1.1
Caching works for sub-domains, e.g. foo.domain.com.
Caching does NOT work for domains, e.g. foo.com.
I am a programmer and can fix this if provided with a brief overview of what could possibly be causing this and/or where to look in the codebase.