In official release, we are outputting \begin{tikzpicture}[yscale=-1]
So we made a new beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
In this beta, we are taking scaling into account, so we are outputting something like:
\begin{tikzpicture}[xscale=1.5,yscale=-1.5]
We have not really tested this, so we are very interested by your feedback.