Media controls in chrome for Mac
If you use the desktop client for OS X grooveshark says they have enabled keyboard shortcuts: 


  http://wanderr.com/jay/keyboard-shortcuts-for-grooveshark-desktop/2010/06/05/


But if you want to use the web player, it looks like it might be a lost unless you use Firefox for which there is an addon named keySharky: 


  https://addons.mozilla.org/en-US/firefox/addon/keysharky/