The release plugin gives a default scm tag to used when doing a release:prepare, continuum's release prepare does not use the same value.
I happen to be on a branch so in continuum my project's scm tag is the branch name. (using cvs).
When i went to release prepare, the tag default was the existing value in the pom for the scm tag. I didn't change it and when the project finished the prepare, there was nothing tagged (because the tag it tried to use was the same name as the branch I was on).
The release plugin uses a different default and doesn't create this problem.