Okay, this actual happened to me in ADT 2007 also, but it's worse in ACA 2008.
Okay, I am in model space with 3 viewports. Left monitor has 2 horizontally with the top one top view, and the bottom one whatever side I'm working on. The other monitor has a perspective.
Here's the example. I'm drawing in the top viewport. I go to draw something in the bottom one, and the UCS is no longer parallel. Imagine I have ortho on and want to draw a line, it will only go side to side, not up and down. I have to "pick" that view again so it regens, then I can draw in that viewport, but guess what, now I can't draw in the other one because of the same problem.
Anyone have that happen and what is the deal with it?