Similar to recommended posts, what about a 'forum' that is a view of the threads most recently posted to? It would avoid trolling round the seperate forums and might lower server load as a result. I'd imagine the the most relevant data would be a list of:
thread title / last post date / last poster / forum
Per-user selection forums of interest (by cookie or user setting) would be an obvious optimsation, though unlikely of great worth if only thread titles were being shown (one line per recently update thread): it might even encourage folk to visit other forums they normally don't when something interesting pops up.