My main motivation was have 1.6 be my default JDK. With Ganymede I had to set my default JAVA_HOME env var to point to 1.5, and point 1.6 dependent apps -- like my command line mvn builds -- to the (non default) 1.6 JDK. That's exactly the kind of thing I forget 5 minutes after I do it.
Just in case I need to do this again: as far as I could tell, upgrading to a major version of Eclipse currently requires a full, clean install. Which means no associated plugins. So I'm writing down the plugins I need to install, where to get them, etc.
I installed the 32 bit Cocoa version of Galileo from http://www.eclipse.org/downloads/download.php?file=/technology/epp/downloads/release/galileo/R/eclipse-jee-galileo-macosx-cocoa.tar.gz
The diff between Cocoa and Carbon and 32 vs 64 bit is explained in detail here.
The tar file unloads to an eclipse directory: make sure you move your old version out of this dir if that's where you have it!
these were in order (Maven required Subversion)
(1) Subversion Plugin: I followed these instructions to install subclipse.
(2) Maven Plugin: I installed from http://m2eclipse.sonatype.org/update.
(1) I needed to change my JAVA_HOME environment var to point to my 1.6 install (I use soylatte).
(2) I needed to upgrade my subversion client to > 1.4 otherwise I saw an 'unsble to launch default SVN client' when trying to browse my SVN repo. I downloaded the latest svn client, restarted eclipse, and all was well.