Where the &%$@ is HashSet?

If you've created a .Net 3.5 project in Expression Blend 2 (December Preview) and then opened it up in VS2k8, you may have noticed that using System.Collections.Generic does not give you HashSet.

The type or namespace name 'HashSet' could not be found.

You may be gnashing your teeth over this, but the simple solution is to add System.Core.dll as a reference to your project because it appears Blend 2 leaves this out.