Aspect-oriented Programming in .NET
The discussion about some of the features of Spring.NET that support Aspect Oriented Programming sounded very interesting.
Looking at their documentation, it appears that you can either do the AOP programmatically or via xml config files.
Neither approach really grabs me at the moment.
I think it would require decent testing as some people have not had 100% success with it, but I do like the approach of using .NET attributes to indicate the advice to apply.