  In this paper we study the interplay of electric and magnetic backgrounds in
determining the decoupling limit of coincident D-branes towards a
noncommutative Yang-Mills (NCYM) or open string (NCOS) theory. No decoupling
limit has been found for NCYM with space-time noncommutativity. It is suggested
that there is a new duality, which we call V-duality, which acts on NCOS with
both space-space and space-time noncommutativity, resulting from decoupling in
Lorentz-boost related backgrounds. We also show that the holographic
correspondence, previously suggested by Li and Wu, between NCYM and its
supergravity dual can be generalized to NCOS as well.
