Delphi 6 includes CLX, so if you don't use stuff specific to a particular OS (e.g. Windows only ActiVex controls) it should be possible to have a common code base with a few (hopefully only a few) differences.
I may be needing to write some configuration programs for serial-interfaced devices, and it'd be nice to have the same code base for both Linux and Windows -- and yes, I would want to support both OS's. I think this could be done in Kylix 1.0 and Delphi 6.0, but won't know for sure until I try it.
Tony