v2.5 method for adding icon to tinymce

Hey,

I’ve just upgraded to wordpress 2.5 rc1 in order to test my plugins (new admin panel looks great btw).

The method for adding an icon to tinymce must have changed or my code is no longer compatible as the icons on the wysiwyg editor for one of my plugins has disappeared!

It took me a while to find the correct code last time and it was a bit of a mashup from code I found in other plugins, does anyone know the best way to do this?