As Tobias Neumann suggested in the international list, it's easy to embed the AWT-based gvSIG canvas in an Eclipse RCP application.
As a proof of concept, I have created a simple mapping application and added gvSIG's main mapping libraries (including some vector drivers, the geometry model and the layer model) as a plugin. You can see the code here:
That does not include the plugin where I have added gvSIG's main JAR files. You can create it easily and add it as a dependency to the main project.
Here is a short video where I am adding a couple shapefiles which are also reprojected on the fly: