  We review evidence that the census of Milky Way satellites similar to those
known may be incomplete at low latitude due to obscuration and in the outer
halo due to a decreasing sensitivity to dwarf satellites with distance. We
evaluate the possible impact that incompleteness has on comparisons with
substructure models by estimating corrections to the known number of dwarfs
using empirical and theoretical models. If we assume that the true distribution
of Milky Way satellites is uniform with latitude, then we estimate a 33%
incompleteness in the total number of dwarfs due to obscuration at low
latitude. Similarly, if we suppose that the radial distribution of Milky Way
satellites matches that of M31, or that of the oldest sub-halos or the most
massive sub-halos in a simulation, we estimate a total number of Milky Way
dwarfs ranging from 1 -- 3 times the known population. Although the true level
of incompleteness is quite uncertain, the fact that our extrapolations yield
average total numbers of MW dwarfs that are realistically 1.5 -- 2 times the
known population, shows that incompleteness needs to be taken seriously when
comparing to models of dwarf galaxy formation. Interestingly, the radial
distribution of the oldest sub-halos in a Lambda+CDM simulation of a Milky
Way-like galaxy possess a close match to the observed distribution of M31's
satellites, which suggests that reionization may be an important factor
controlling the observability of sub-halos. We also assess the prospects for a
new SDSS search for Milky Way satellites to constrain the possible
incompleteness in the outer halo.
