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't

http://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.

That will be crummy if that is the way around this. I have moved away from using multiple stylesheets and IE hacks to a graceful degradation model. My company no longer supports IE6 so the days of having multiple stylesheets was supposed to be over.

Personally I agree with you on that point, slideaway. I find browser-specific stylesheets to be a maintenance nightmare, but there are plenty who disagree.

Anyway, to your original point: PIE's approach to IE9 is still unfortunately very much up in the air. I haven't even had time yet to determine what bugs are present, much less how to approach IE9's ability to render a subset of the same CSS3 properties. This will definitely be ironed out by the time IE9 is released, but I understand that can be frustrating for web devs trying to get ahead of the game.

Out of curiosity, if you disable PIE completely (comment out all the behavior properties), how does IE9 perform natively with your CSS3? Any big holes in its support?