MySQL takes this approach too. 5.5 went GA on 5.5.8 I believe. It can be a little confusing at first if you're used to alpha/beta/rc candidates but it makes more sense over time since you have a consistent numbering scheme vs. something like 1.1-beta2.
I'm not an expert on this but I'm guessing with tomcat being so ubiquitous they do an initial release of the new version, and then it takes a few revisions out in the wild for them to deem something production stable. I think its the equivalent of doing release candidates ala Firefox but without the RC label.
"This is the first stable release of the Tomcat 7 branch.".
I'm a little confused by Tomcat's version numbering:
http://tomcat.apache.org/whichversion.html
Shouldn't 7.0.0 instead of 7.0.6 be the first stable release?