I just installed IE9 and looked at a site I recently launched that uses PIE extensively. I use rgba a fair bit in the layout and what I noticed was that in some places it is working while in other spots it isn'thttp://www.realtyworldwichita.com/
The follow us dropdown is transparent, as is the menu, but the background to the nav and the quicksearch is gone. Also it seems to be wiping out background declarations for anything that uses PIE.
I tried to set the compatibility mode to render back to IE7 or 8, but no dice on that.