I won't repeat the details here but you can check out the news over at the EclipseLink Team Blog and at the GlassFish Aquarium.
This is great news for JPA 2.0! And since EclipseLink shares the same source code "DNA" as TopLink Essentials, the JPA 1.0 reference implementation, migration for users of TopLink Essentials won't be difficult. Especially since EclipseLink includes tools for migrating code and configuration files from TopLink Essentials and Oracle TopLink.
IMHO, moving to EclipseLink is a no-brainer. EclipseLink has all the functionality of TopLink Essentials plus advanced features like cache coordination and advanced mappings like Converters , Returning Policies, and Basic Collections. Check it out!