Tmonews has leaked a T-Mobile retail partner sales FAQ in which it is explicitly states that T-Mobile will never again stock the Nexus One in their retail stores.
This would seem to be the last nail in the coffin for the Nexus One’s US tour, though it is still sold abroad in the UK and South Korea. While the document doesn’t explicitly preclude the possibility of online sales, “The Nexus One was sold and marketed by Google” is pretty damning.
Also, this document is addressed towards brick-and-mortar retailers, and thus it wouldn’t be relevant to mention T-Mobile’s online sales policy. I think it is safe to assume, though, that you won’t be seeing the Nexus One on T-Mobile’s web store anytime soon.