A member of an rvalue expression is an rvalue.
The standard states in 5.3.5 [expr.ref]:
If E2 is declared to have type
“reference to T”, then E1.E2 is an
lvalue […]
– If E2 is a non-static data member, and the type of E1 is “cq1 vq1 X”, and
the type of E2 is “cq2 vq2 T”, the
expression designates the named member
of the object designated by the first
expression. If E1 is an lvalue, then
E1.E2 is an lvalue.