(imported from http://blogs.codehaus.org/people/avasseur, read comments there)
While AOP has reached a decent maturity level in the Java platform, thanks to several iniatives from different players with different priorities and goals (IBM, BEA, JBoss, Spring, and individuals like Bob Lee to name a few), Microsoft seems to be willing to move beyond that.
Last year at AOSD they were hosting a session on Phoenix, an interesting building block in the .Net CLR labs to help enable AOP frameworks. The community in .Net around AOP is fairly active as well, and Microsoft did awarded some of the projects, like Alan Cyment' SetPoint hosted on CodeHaus, the home of AspectWerkz.
Recently (Nov. 2005) Microsoft has set up a research event to emulate discussions around the best way to have good AOP solutions in the .Net platform. Several key .Net AOP project leads and folks from the AOP research academia were invited for a full day event at Redmond.
As far as I know Sun never set up such an event to try to understand AOP from its roots, and most of the discussion on the field has been driven by individual to individual networking and the AOSD annual conference.
I am eager to see where .Net ends up in that field in 2 years from now. There are certainly interesting architectures and concepts that we have implemented in Java based AOP that could be ported rightaway to .Net, though .Net luminaries have certainly several new ideas that 'd be worth exploring from Java luminaries point of view - and possibly standardizing on - obviously outside the JCP - unless Sun suddenly cares more about AOP.