Custom Validation via Action Filters

At the end of last week, I was tinkering with the Entity Framework 4.0 and MVC3;  MVC which we’ve been using a long time here and we all know and love, Entity Framework which quite frankly feels unfinished unless you have the CTP5 patch so that one can write true code-first with POCO’s.  Now I know it’s illegal to use CTP patches on production environments, but apparently this is the last CTP release before a full release first quarter this year, so we can begin prototyping with it at least.  Anyway getting to the point, during my tinkering, I think I may have worked out a means of custom validating objects in the controller, setting ModelState, and creating errors well before a controller method is invoked, essentially creating a validation layer that at run time sits between a UI post and the controller.  I'm sure I'm not the first who's done this, and this has been worked out and blogged a gazillion times before but I figure I'll have my say on the matter...