Proposed resolution:

Change [futures.unique_future]/3:

The effect of calling any member function other than the destructor, or the move-assignment operator, or valid, on a future object for which valid() == false is undefined.

Rationale:

Solved by N3058.

[ 2010 Pittsburgh: ]

Moved to NAD EditorialResolved. Rationale added below.

[ 2009-12-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[futures.unique_future]/3 says:

The effect of calling any member function other than the destructor or the move-assignment operator on a future object for which valid() == false is undefined.

This means calling future::valid() is undefined unless it will return true, so you can only use it if you know the answer!