There are three packages implementing the Java Activation Framework in
Debian. They could be removed now that Java 6 and upward integrate the
most recent version of JAF.
Even if default-jdk package will soon become alias for openjdk-7 [1], for now it's a mix of openjdk-6 on some platform and GCJ (vaguely Java 5 compliant) on others.
It means that if someone try to build a package which use JAF (and so Build-Depends only on "default-jdk") it will fail on platform using GCJ. For this kind of needs, I think we could use a versionned Build-Depends on "default-jdk (>= 1:1.6)".