Hi Richard,
Both the news and regular forum activities appear to use the same Moodle code, so if I'm not mistaken, the only way you could get them to use different icons would be to make changes to the forum code, plus provide a new icon. Since the forum is built in to Moodle, that means you would need to make changes to Moodle core code, which is generally not a good idea as you would need to propagate your changes forward into each new version of Moodle. Granted, you are using an older version, so that might not be as important to you as it might be to others.
One other possibility might be to make a clone of the forum code, and change it's name from forum to something else. I've done this myself, just last week, for another project I'm working on. With something like Notepad++ it is fairly easy to find and change every occurrence of 'forum' in the code.