On April Fools' Day, amongst our extensive HKO coverage, Daniel Whitcomb reported that the combat log changes introduced in Patch 2.4 were being rolled back in Patch 2.5. Slouken has since indicated that he was "just kidding", as Daniel had guessed.
So the combat log will not be rolled back and there may not be a Patch 2.5 before Wrath of the Lich King.
He did say, however, that the UI changes that were to happen in the fictional 2.5 will in fact happen in WotLK. Specifically, the arguments "this" and "argN" will be phased out of the Blizzard UI in favor of "self" and other local arguments. Slouken recommends that authors start using "self" in the meantime, so that they will be prepared for when the expansion comes out.