  A neutralino relic density in accord with WMAP measurements can be found in
the minimal supergravity (mSUGRA) model in several regions of parameter space:
the stau co-annihilation corridor at low m_0 and the hyperbolic branch/focus
point (HB/FP) region at large m_0 at the edges of parameter space, and the bulk
and Higgs boson funnel regions within. In the regions at the edge, the mass gap
between the next-to-lightest SUSY particle (NLSP) and the lightest SUSY
particle (LSP) becomes small, and backgrounds from $\gamma\gamma\to f\bar{f}$
($f$ is a SM fermion) become important for NLSP detection at an $e^+e^-$ linear
collider. We evaluate these backgrounds from bremsstrahlung and beamstrahlung
photons, and demonstrate that these do not preclude the observability of the
signal for the cases where either the stau or the chargino is the NLSP. We also
delineate the additional portion of the stau coannihilation region, beyond what
can be accessed via a search for selectrons and smuons, that can be probed by a
search for di-tau-jet events plus missing energy. The reach of a LC for SUSY in
the HB/FP region is shown for an updated value of $m_t\simeq 180$ GeV as
recently measured by the $D\O$ experiment.
