STL TR1 type_traits surprises

On using the new TR1 type_traits I was surprised to find that floating point numbers fail the is_signed test. This was not what I expected, seeing as floating point numbers do negative numbers quite well. It seems that this test only applies to integral types.

Has anyone else come across a type trait test that did not do what they initially expected?

Possibly it simply means that there are no "signed/unsigned" variants of the type. As in, you can't have "unsigned float" as a type.