Well, I have found that in 4E as a DM I change targets a lot more often than in 3E because the fights are much more dynamic. Often the tactical situation changes completely from one round to the next.
For reasonably intelligent foes, I typically adjust their priorities based on what the pcs actually did in the encounter. If a PC manages to land a lucky crit, they're likely to focus on him for a while.
At the beginning of a combat I typically picked pcs that exposed themselves (most often this would be the melee striker(s)). When they noticed a leader patching up a striker, they'd focus on the leader.
I find it amusing that my monsters quite often become confused or enraged by not being able to settle on which target to concentrate on. My party has 3 strikers, 2 leaders and 1 defender, and they can ALL do things that trigger a "that enemy must die!" response. All in all I find the cleric is often the most obvious choice for monsters as he has a lot of very big, very flashy burst attacks, as well as keeping the rest of the group bouncing off maximum HPs.