Jeroen in the comments say that Microsoft is watching what the industry does in AOP… But to be really honest, Microsoft was one of the first in the industry to provide interception and decoration of code with its COM+ Services. You can ask Clemens about his experience hooking up in this COM+ pipeline! (I was looking for posts about it, not sure he ever put down on the net what he explained in one conference I was at in Paris).
Moreover, I think it’s fair enough to say that the interception layer has been implemented in the .net world, through the use of the TransparentProxy / RealProxy couple. What prevents that from being used more efficiently? It’s as slow as it gets. It needs a context on its own. Some people (I’m not putting names as I can’t assert for sure, but I have my opinion on the matter) pushed to have call contexts instead of object instances contexts, which results in it not to be optimized before .net 3 or 4, maybe (according to this guy which should known what he’s talking about). And that’s a shame.