streaming AND file-based log-shipping?

The cookbook text says, log shipping "is mostly superseded by streaming
replication in 9.0, though is still useful as part of a comprehensive
backup strategy."  The implication is that the WAL archive might be
needed for recovery in some cases:

1. Under what circumstances might the WAL archive come into play in recovery?
2. If the streaming rep fails for some reason will the standby begin
   recovery from the WAL archive, assuming the WAL can still be shipped?
3. What query or command set can be used to tell if file-based recovery is
   configured correctly if streaming replication is active?

I've configured both (I think), but it isn't clear to me whether
the file-based part will function.  I guess I could bust the stream somehow
and see what happens, but I can't find this idea documented.