The missing support for unique keys is well known limitation of Entity framework 4. Few days ago, the Entity framework team has published the article which I take as announcement of unique keys support in the next version(s) of Entity Framework. After announced support for table-valued functions, it is the second nice feature targeting the next version of Entity framework announced this year. Just be aware that "the next version" does not mean Entity Framework 4.1 (the code-first).
Update: Unfortunately this feature will not be available in the next version. ADO.NET team didn't include the feature into .NET Framework 4.5 and we will have to wait at least till a next major release (.NET Framework service pack or next version).