Holy cow, I wrote a book!
Eric Lippert has posted Eric's
Complete Guide to BSTR Semantics. This is a document that is widely valued within
Microsoft, since BSTR semantics are rather tricky, and I'm glad he's willing to share
it with the world.
In particular, Eric spends quite a bit of time discussion the very important equivalence
of NULL and L"" in BSTR semantics, something that is buried
in paragraph seven of an MSDN document you probably didn't even bother reading.