After following the instructions for doing a clean build of Maven (pulled from CVS) I appealed to the group for help.
What I was missing was
set MAVEN_OPTS=-Xmx512m -Xms12m
As an environment variable. Without this, runs out of memory. With this ... succesful build.
The documentation should be updated to discuss this, especially on the "building from source" page.
Fixed in CVS today