Exporting iTunes smart playlist definitions
If you select a smart playlist, then choose File > Library > Export Playlist... and choose the XML file format, you can then use the corresponding Import Playlist... command from another iTunes library to import the smart playlist definition. 

Unfortunately, iTunes only lets you select one playlist at a time, so there's no way to select multiple playlists and export them at the same time. You can do the "Export Library..." which does everything, but then you have all sorts of extra stuff you don't want. 

If you're comfortable hacking around a bit though, you could export the whole library, then use PlistEdit Pro to edit the file and remove all the playlist and track dictionaries that you don't want, leaving just your smart playlists. Then you'd have all the playlist definitions in a single file that you can import into another iTunes library.