Hi,
It could be a very interesting and helpful feature. I will add it to our future plans.
In the meantime I recommend using reflection in your unit tests to query the function's metadata and check if a certain attribute is attached to it
Update - It seems that NUnit 2.5 has a new feature that does exactly that:
[Serializable]
public class testclass {}
[Test]
public void Test()
{
Assert.That(new testclass(), Has.Attribute<SerializableAttribute>());
}