I have the following profiles in my parent pom
P1
true
In general, Maven profiles are not inherited (see http://jira.codehaus.org/browse/MNG-5127 for a discussion and links to blog posts that might be useful). I've had success doing something like this:
P2
${project.basedir}/src/main/whatever
P2
${project.basedir}/src/main/whatever
Also I think that P1 will not be active if P2 is. This is because is true for P1. The element name is a little misleading in my opinion. "Active by default" implies "always active" when it really means "active only if no other profile in this POM is active."
The above discovered using Maven 3.0.x.