How compatible is code written under Solaris with Open Solaris ? I would be interested specifically in some kernel modules .
I think it is hard to quantify software compatibility, but I'd say code written for Solaris is quite forward compatible with OpenSolaris kernel. OpenSolaris source code evolves into what will be Solaris 11, and Sun's commitment to backwards compatibility is quite a fact.
Kernel modules in particular will be very compatible between Solaris and OpenSolaris. OpenSolaris (via Project Indiana) is evolving the user-space components more heavily, including the installer and packages.
Kernel modules written for Solaris should function in OpenSolaris following a simple recompile providing you are using the exposed kernel APIs that are compatible between the releases that you are using in Solaris and OpenSolaris.
There is a huge amount of work in Sun to ensure that programs written using publicly exposed interfaces are compatible. There is a listed 'Exposure/Stability' entry at the bottom of manual pages for most APIs that state in defined terms how someone can use it.