<p>Well, I must be brain-damaged, because I can't find the java source for Sun's persistence.jar or JBoss's ejb3-persistence.jar JPA package.  They <em>are</em> open-source aren't they?</p>

<p>I looked all over the java.sun.com site as well as the GlassFish wiki, but came up empty.</p>

<p>I'd like a src.zip or folder like Sun delivers with Java JDKs.</p>

<p>Of course, I <em>really</em> don't <em>have</em> to have it, but I think it's fun to browse the source once in a while.  And it helps me to debug my code sometimes.</p>

The ejb3-persistence.jar contains the JPA API (javax.persistance.*) implementation which specified as part of the EJB 3.0 JSR 220 specification (the upcoming specification for JPA is JSR 317 which is now separate from EJB 3.1 spec)
The reference implementation is the one included with the Glassfish, I believe the one used in JBoss is the Hibernate implementation so you should be able to find the source code there.  You are free to use the one whose license satisfies your need (if you need to redistribute) although I believe they are all redistributable. For an Apache style license I recommend the one used by the Apache OpenEJB.org project (provided by the Apache Geronimo project).
You can browse the source here:
https://github.com/apache/geronimo-specs/tree/trunk/geronimo-jpa_2.2_spec/src/main/java/javax/persistence