This post follows this wanted feature : http://plantuml.sourceforge.net/qa/?qa=4682/param-to-center-text-in-activity&show=4684 delivered in V4085.
I think it would be nice to apply skinparam defaultTextAlignment to other activity diagram items (if, repeat while...)
In last beta: https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
We have extended skinparam defaultTextAlignment to if, else, repeat and while nodes.
However, this has not been highly tested, so please feedback if you find other issues!