Realistically the increase from 20 to 30 yards for Taunt is based on bringing it into line with Hand of Reckoning. However, there has been some interesting discussion on the tanking forums regarding whether this was tied into upcoming encounter designs or could be utilized on tauntable mobs to trivialize fights.
Two Protection Warriors with Vigilance on each other stand 30 yards apart, and DPS/Healers make the 3rd point of a equilateral triangle 30 yards away from the two tanks. The boss/mob would be taunted between the two tanks reducing the dps output of the boss on each tank to (8 seconds - 2 * (time to travel between the tanks before attacking)). If DPS/Healers exceed threat, then the travel time to their group would ideally be enough time for one of the tanks to taunt back.
Is it viable? Don't know, haven't tried it. Vigilance would be nice for the damage reduction, but would decrease the threat ceiling minimally for DPS. Other tanks could be used, but wouldn't have the vigilance option. Interesting theory, but whether it will ever be a boss-fight mechanic is debatable.
My other thoughts would be a MDing hunter assigned to each tank to boost the leap-frogging threat levels. The only situation where this would be worth while would be a fight where the bosses' dps output is too consistently high or 'bursty' for healers to heal through.