Next - the property Grid tries to bind "Error" (I know I can fix this in code behind - but this would break my so far "no code behind" MVVM design) - but this isn't very important.
Last not least I implemented "Is Valid" like in the following snippet.
I further know that I could decorate my object with attributes - but the reason to use your Property Grid is that the object is large (a lot of properties) and I don't want to define all these things.
And it would be hard without touching the Linq2Sql autogenerated code.
To do this, on the Data tab, in the Data Tools group, click Data Validation | Circle Invalid Data.