I'm not sure if this is an intentional change that I've missed or not, but running something like:
will retrieve the metadata from the repository on every build:
Downloading: http:9 KB downloaded at 17.1 KB/sec
This is in contrast to Maven 2.2.1 which always continues to use the first version encountered until you use -U:
While that remains in the super POM of Maven 3, it seems that the metadata is retrieved regardless.