DataOpener knows how to open things in the correct application... i.e. emails in thunderbird, word documents in Word etc. etc

This is damn tricky. 
GnowsisAlpha had a dirty C hack done by Leo. This is clearly nasty.

Options: 

 * Apple Java Extensions does this on MacOSX http://developer.apple.com/documentation/Java/Reference/1.4.1/Java141API_AppleExtensions/api/com/apple/eawt/ApplicationListener.html
 * Java 1.6 does this - http://download.java.net/jdk6/docs/api/java/awt/Desktop.html
 * The Java Desktop Integration Classes (jdic) project does this. https://jdic.dev.java.net/ 
   * it uses JNI and native compiled things on every platform
   * Gnowsis already uses this for the trayicon support. 
   * MacOSX support is ropy... at best - only older versions available and only half the API. 
 
 * This has various links: http://wiki.java.net/bin/view/Main/NativeHooks

Solution:

 * Use Apple Java Extensions for Macosx and jdic for all other platforms.