  We extend the lightcone worldsheet lattice approach to string theory,
proposed in 1977 by Giles and me, to allow for coincident D-branes. We find a
convenient lattice representation of Dirichlet boundary conditions, which the
open string coordinates transverse to the D-branes satisfy. We then represent
the sum over all planar open string multi-loop diagrams by introducing an Ising
spin system on the worldsheet lattice to keep track of the presence or absence
of fluctuating boundaries. Finally we discuss a simple mean field treatment of
the resulting coupled Ising/coordinate worldsheet system. The interplay between
Neumann and Dirichlet boundary conditions leads to a richer phase structure,
within this mean field approximation, than that found by Orland for the
original system with only Neumann conditions.
