Looks like Minecraft 1.7 may include a bunch of really interesting changes: http://www.minecraftwiki.net/wiki/Upcoming_features
Among them are pistons, a new lighting engine that doesn't require chunk updates (less server lag), and a new (unannounced) mob. If you haven't seen what pistons (might) be capable of, check out this video of the piston mod before it was integrated into Minecraft. The MC devs have stated that they've changed pistons slightly from what the mod includes, but the basic functionality should remain the same.
Notch has stated that 1.7 will be released sometime after E3, so it is most likely to appear sometime toward the end of this month or in July.