Simple enough really - just a selection box showing available forums (based on users privs), with an option to highligh one/several/all. Any that are highlighted are removed from that persons What's New list.
I imagine a link in the user drop-down: What's New Settings
Linking to a page, similar to alert preferences, where you have a selection box and a save button.
This is something I would like done ASAP if possible as it is a key feature for some of my members and I'd rather underestimated its importance prior to migration ...