This is a highly-compatible release which everyone using GlassFish 3.0, 3.0.1 or 3.1 should really consider. Its content were for the most part driven by community and customer requirements. Some had to do with critical bug fixes (we have hundreds of fixes in this release), others with upgrading the versions of the product components such as Weld, EclipseLink, Jersey, etc. with performance also a focus to keep GlassFish in the top performing spot.
Another important driver for this release was support for additional platforms : AIX, 64-bit load-balancer, and maybe most importantly support for Java 7 (shipping today as well). This means that you can now benefit from the JVM improvements that come with this new shiny version but also use the new language constructs defined in project coin. Check out this screencast for an example of what can be done.
You can also listen to this new episode of the GlassFish Podcast which features a discussion with Sathyan, the Engineering manager for this release. Finally, you can get more details in the release notes and the documentation set.
If you are an existing GlassFish 3.x user, the update center will offer you a binary upgrade, similar to what happened when 3.1 was released in February.
In addition to the Open Source bits, GlassFish 3.1.1 is available via Oracle GlassFish Server, the Oracle-branded and supported offering, as well as in the Java EE 6 SDK Update 3 (including a bundle with JDK 7).