Advice on POCO Validation with ASP.NET MVC/Entity Framework

Here's the scenario:

• ASP.NET MVC2 Web Application
• Entity Framework 4 (Pure POCO's, Custom Data Context)
• Repository Pattern
• Unit of Work Pattern
• Dependency Injection
• Service Layer mediating Controller -> Repository

So basically, all the cool stuff. :)

Flow of events for a basic UI operation ("Adding a Post"):

1. Controller calls Add(Post) method on service layer
2. Service layer calls Add(T) on repository
3. Repository calls AddObject(T) on custom data context
4. Controller calls Commit() on Unit of Work

Now, i'm trying to work out where i can put my validation.

At this stage, i need two types of validation:

1. Simple, independant POCO validation such as "post must have a title". This seems a natural fit for Data Annotations on the POCO's.
2. Complex business validation, such as "cannot add a comment to a locked post". This can't be done by Data Annotations.

Now, i have been reading "Programming Entity Framework, Second Edition" by Julie Lerman (which is excellent BTW), and have been looking into hooking into the SavingChanges event in order to perform "last-minute" validation. This would be a nice way to ensure validation always happens whenever i do "something" (add, modify, delete), but it's also a little late IMO (as the items are already in the state manager) - so what can i do if validation fails, remove them?

I could of course make my POCO's implement an interface (say "IValidatable"), and call a method on this interface during this event.

But this seems "too late" for business validation - is this