Export simply saves the iTunes information (Name, Artist, Album, etc.)
Okay, I did a bunch of experimenting for this.
Condition: I am using iTunes 7.5. Maybe this functionality was removed or changed in later versions.
Selecting by highlighting doesn't produce the desired results. However, using File > Export in iTunes will export a whole playlist to a text file. I find this does indeed save more than just basic artist, etc. information. It saves a very long list of items including the location of the file.
If I then create a new library by holding down the option key while starting iTunes, and then use File > Import to import the exported playlist it will create a functional playlist in the new library as well as adding those tracks to the library. Of course things such as playcount, etc are not carried over.