  Noncommutative U(N) gauge theories at different N may be often thought of as
different sectors of a single theory. For instance, U(1) theory possesses a
sequence of vacua labeled by an integer parameter N, and the theory in the
vicinity of the N-th vacuum coincides with the U(N) noncommutative gauge
theory. We construct domain walls on noncommutative plane, which separate vacua
with different gauge groups in gauge theory with adjoint scalar field. The
scalar field has nonminimal coupling to the gauge field, such that the scale of
noncommutativity is determined by the vacuum value of the scalar field. The
domain walls are solutions of the BPS equations in the theory. It is natural to
interprete the domain wall as a stack of D-branes plus a stack of folded
D-branes. We support this interpretation by the analysis of small fluctuations
around domain walls, and suggest that such configurations of branes emerge as
solutions of the Matrix model in large class of pp-wave backgrounds with
inhomogeneous field strength. We point out that the folded D-brane per se
provides an explicit realization of the "mirror world" idea, and speculate on
some phenomenological consequences of this scenario.
