I have a simple Paint Effects tree-like object (I've simplified it to just branches- no twigs or leaves or anything...it's basically a bunch of tubes...)
(I'm using Maya 2009 Unlimited on OS X 10.5.6, Mac Pro Quad-Core Nehalem 2.93Ghz, with 3GB of RAM.)
I've applied expressions to some of the Tubes Behavior Attributes (Bend, Spiral, Turbulence) to achieve some writhing activity. It works perfectly, but when I try to convert the Paint Effects object to NURBS, it only converts the object on the frame I am on - and then there is no animation whatsoever.
The same thing happens if I try to convert to Polys, or even if I bake the expressions into actual keyframe / curve data - no animation on the resulting NURBS or Poly shape.
It DOES maintain the animation if I convert the Paint Effects object to Curves, though - although this is not ideal, since I am not getting the same "performance" if I use those curves to drive IK Spline deformation of the resulting NURBS shape.