If I remember correctly, one of the P7 reason was the improved scene preview.  So, how much is it an improvement over P6?  Any screencaps comparing those two?  Can material room nodes, such as blender, colormath, and etc, be previewed in preview window as well?

Thanks, I appreciate it.

I don't see any noiticeable improvement in preview window, shader nodes like you're asking about do not display in preview.  Certainly Blender and Color_Math do not, I've tried both of those.

Ah, that's rather disappointing.  Was really looking forward to see some improvements there.

Thanks.

By default the preview is not different from P6 but with the right graphics card and when the hardware shading is turned on (on the preview shader settings) the nodes do display on preview.
That doesn't mean the preview shows everything and shows it correctly. Some materials don't show (textures don't always show). Point lights don't show properly (actually lights give a hint about their presence and are not actually very accurate).
Overall preview has improved and after fixing some issues it will probably be even better but it remains a preview.

Dang.  I want shadows or ambient occlusion so i can use the preview as final output!  (Thoughts of nice new TrueSpace preview...)

The preview is better - you just need to enable that... Render -> Render Settings -> Preview Tab...