XtremeSystems Forums user Sampsa published some results of a Radeon HD 4870 X2 microstuttering article he's working on. He released a graph from the game Race Driver: GRID that shows how bad micro-stuttering is on the Radeon HD 3870 X2, GeForce 9800 GX2, Radeon HD 4870 CrossFireX and the upcoming Radeon HD 4870 X2.
The preliminary results indicate the Radeon HD 4000 series have a lot less problems with micro-stuttering than the Radeon HD 3800 series, it's still there but the delays are a lot smaller. Optimally the line should be entirely flat but on the Radeon HD 4000 series the frame delays are just a couple of milliseconds while on the Radeon HD 3800 series you get huge delays of almost 20ms.
The rumours that the Radeon HD 4870 X2 would suffer less from this issue turns out to be false as there's almost no difference between the dual-GPU card and two Radeon HD 4870s in CrossFireX. Whatever ATI did to minimize the issue, it's not just present on the Radeon HD 4870 X2.