There is surprisingly little information on the internet regarding what Moonlight is exactly from a technical point of view. Is it correct to understand it as Mono-based CoreCLR + base libraries (the same .dlls as Silverlight has as part of the plugin itself)?
Does this mean that if I have Silverlight 3.0 application and I reference base library, plus develop my own Silverlight libraries, this application will work the same in Moonlight 3.0 (when available), except for bugs, etc., of course.
Yes, your Silverlight applications should run with moonlight (modulo bugs, yeah). From moonlight-project.com:
Moonlight is an open source implementation of Silverlight, primarily for Linux and other Unix/X11 based operating systems. In September of 2007, Microsoft and Novell announced a technical collaboration that includes access to Microsoft's test suites for Silverlight and the distribution of a Media Pack for Linux users that will contain licensed media codecs for video and audio.
Moonlight 2.0 should be out soon and there's already work underway in supporting Silverlight 3.0 features.