Less SQL querys used, could be used on index.php, as the forums hardly ever change. Could also be used on the User list thing the administrator sees, could also be used on profiles. These are just ideas.
Cache update could be around once a day for forums and once an hour for the userlist/profiles
are you having performance issues?
this could be offered as a patch/add-on, but i'd never put it in trunk.
No I'm not having performance issues but for sites who host ttf on a really slow SQL server it could be quite useful.
You can just use memcache? It requires some small modifications to the code, but not too many depending on how the code was written ... (for vBulletin for example it's only 20 or 30 lines).