An excellent reply, it is something we didn't really think about until recently. Our problem is an engineering preference for isolating a voltage into several islands with a pi filter between each.
Having had some outside advice from one of our partners, about the problems this caused, they did recommend having the ground plane between the split plane and the signal would ease the problem.
Reading your reply and Henry Ott's 'Electromagnetic Compatibility Engineering' (P627-630 are good) adds more things to think about with high speed design.
I am still unsure about striplines, as H. Ott only mentions adjacent layers, if you have two contiguous ground planes above and below, the signal will couple to these planes with no E or H fields extending beyond the planes?
I would be interested in any information or data on how much noise and problems this adds.
More food for thought.