It adds menu items to the File->Import and File->Export, which can be used to make toolbar buttons if you prefer. You'll need to change the lines that refer to 'folder = ' from my doc folder to your own.
Use at your own risk. Read the script before use. It will not work until you modify it for your own system.
Edit: Minor script/zip update. Import to Softimage still needs 3DC to generate the OBJ/MTL files.
Edit 2: Fixed import back to Softimage (thanks Andrew for the help!). Also, the names of the applink files are "project-scene.*" instead of always using the same names for all files. Finally, images are imported into the project via 'CopyExtFilesIntoProject' command so if you don't want that you'll need to disable that in the script. It leaves the unused-after-CopyExtFilesIntoProject images in the exchange folder because there is no easy way to safely clean that up and it shouldn't really affect anything. Feel free to clean up the folder whenever you get the urge.