This blog post from Vince is from December 2011, so it looks like we already have GlassFish 3.1.2 support on Eclipse for a long time.
Recent changes in the GF 3.1.2 code did make it necessary to release a new version of the plugin, since users could not register recent builds by claiming that it was a 3.1.1 build any longer.
I just finished pushing an update that makes 3.1.2 an explicitly 'supported' run-time to address that issue.
I have also finished the work that Ludo started to convert the plugin to a Maven/Tycho based build.
This is the first release of the plugin that was built and pushed onto java.net via hudson jobs, thanks to the wonderful support of Jane Young.
You can read more about the update on Kraemer's blog post.