I spent an hour or so watching old trailers last night and I noticed that around about 2004-2005ish, the trend of having the "in a world" guy explain the story dropped off dramatically. There are still plenty of trailers after this that have the text on screen (y'know it fades in and out between shots) but the latest trailer I can find that still has the voiceover is Pan's Labyrinth in late 2006.
I expect there to be more after this and I expect it to last in the comedy genre for a little longer than others, but is there a definitive point where they just stopped having a voice over on big trailers?