If this nice behaviour is mandatory in 2.0, could you provide a
pointer to the part of the spec saying so? (I couldn't find it.)

http://www.w3.org/TR/xslt20/#document

[quotes snipped]

then

document('test2009031701.xml#f1')

is explictly defined to work like

document('test2009031701.xml')/id('f1')

in XSLT 2.0

Thanks. I had read the parts of the spec you quoted but hadn't
understood how they would explictly define the former invocation
to work like the latter. But I believe you they do, of course!
(And I can see some implementations do so as well.)