depends on what toolkit of you are using. If you are doing X programming itself then yes you will probably be able to keep a few things (as most X implementations are similar).
If you are using a toolkit such as GTK then yes you will be able to keep the experience because they are the same on all platforms.
Why would you want to do X11 development anyway? Why not just use a cross platform toolkit like QT?
In fact, after giving this more thought, why not just use a proper cross platform language like java, that way you won't have to know anything about any of the platforms you are using. Netbeans has come up in leaps and bounds with GUI development with swing.