loading packages conditionally

The OpenMx repository has a work-in-progress feature that generates graphs from RAM style models. This package requires the Rgraphviz library. I don't want to add Rgraphviz as a required library. Not all users will need to make graphs, and it turns out that the Rgraphviz library can be a pain to install on Windows (you need a specific version of the Graphviz library that is hidden on their website). So this feature conditionally works if the Rgraphviz package is installed. I've added some runtime tests to zzz.R that conditionally create S4 classes that are subclasses of classes defined in the Rgraphviz package. The odd thing is that R 2.9.1 on Windows does some kind of static analysis and believes the Rgraphviz package is required. So I see a message "loading Rgraphviz" package when the OpenMx library is loaded, even when Rgraphviz is not installed. Odd behavior from R, just wanted to appraise the developers of the matter.