  Poynting-Robertson drag has been considered an ineffective mechanism for
delivering dust to regions interior to the cool Kuiper belt analogues seen
around other Sun-like stars. This conclusion is however based on the very large
contrast in dust optical depth between the parent belt and the interior regions
that results from the dominance of collisions over drag in systems with
detectable cool belts. Here, we show that the levels of habitable zone dust
arising from detectable Kuiper belt analogues can be tens to a few hundreds of
times greater than the optical depth in the Solar Zodiacal cloud. Dust
enhancements of more than a few tens of `zodi' are expected to hinder future
Earth-imaging missions, but relatively few undetectable Kuiper belts result in
such levels, particularly around stars older than a few Gyr. Thus, current mid
to far-IR photometric surveys have already identified most of the 20-25% of
nearby stars where P-R drag from outer belts could seriously impact
Earth-imaging. The LBTI should easily detect such warm dust around many nearby
stars with outer belts, and will provide insight into currently unclear details
of the competition between P-R drag and collisions. Given sufficient confidence
in future models, the inevitability of P-R drag means that the non-detection of
warm dust where detectable levels were expected could be used to infer
additional dust removal process, the most likely being the presence of
intervening planets.
