This "warning" in Eclipse had me baffled on exactly what it was wanting and more specifically on where this setting was that needed to be modified.
Build path specifies execution environment JavaSE-1.6. There are no JREs installed in the workspace that are strictly compatible with this environment..
So I thought this is perfect additional documentation is definitely needed out on the net, so here is my chance to add to it, and hopefully help someone in the future when they find this cryptic message. I had just been making the change over in my Eclipse RCP project to use Java 1.7 instead of Java 1.6, when this Warning from Eclipse started showing up in the my Problems View. The solution was to access the properties for that project, change to the Java Build Path, and then to the Libraries Tab, and remove the 1.6 libraries and add the the 1.7 libraries.
Simple but a bit cumbersome to figure out sometime, at least when your working a deadline.