Dubious test in /cast/

Most of them I agree with, but this one seems wrong:

http://www.w3.org/2001/sw/DataAccess/tests/data-r2/cast/manifest#cast- 
str

data:

:iri :p :z .
:str :p "string" .
:fltdbl :p "-10.2E3" .
:decimal :p "+33.3300" .
:int :p "13" .
:dT :p "2002-10-10T17:00:00Z" .
:bool :p "true" .

query:

SELECT ?s WHERE {
     ?s :p ?v .
     FILTER(datatype(xsd:string(?v)) = xsd:string) .
}

results:

?s
--------
:iri
:str
:fltdbl
:decimal
:int
:dT
:bool

The implication of the first result is that it's legal to case a URI  
to an xsd:string with xsd:string(). I agree with the remainder of the  
test.

It was approved here: http://lists.w3.org/Archives/Public/public-rdf- 
dawg/2007JulSep/att-0087/14-dawg-minutes.html

- Steve