Mike Taulty has a very nice post on timestamps and concurrency in the Entity Framework. Something that I very much like, however, is how he uses all the information available in a single .edmx file, which includes the storage, mapping and conceptual models, to pull information from the storage level and make it relevant at the conceptual level. You can imagine populating other annotations to be consumed or doing similar massaging as necessary.
And all with a concise, declarative bit of programming. Nice work!
ADO.NET Data Services plug: yes, when a service is based on an Entity Framework ObjectContext, custom annotation will flow through and clients will be able to see them when they issue a request for service-root/$metadata.