As far as I can tell, the vast majority of the forums are largely edition-proof, which seems to be a good thing for most people and a draw to the site in general. The most notable mechanics-heavy exceptions are D&D Rules and D&D 4th Edition.
I could see this being a useful breakdown and (more importantly) requiring minimal work to implement:
1) D&D Rules is renamed to D&D Past Edition Rules.
2) Create a new forum called D&D Rules (for 4th edition since it's becoming the current version, and blank slates are kind of cool).
3) D&D 4th Edition is renamed to D&D 4th Edition Speculation Archive and moved to a sub-forum of either D&D Rules or the Archive.
The speculation is interesting and totally worth keeping, but it'll no longer be a current topic once 4th is actually out. It's got a built-in expiration date.
-blarg