Howard et al. The bottom line is that they have made as careful a statistical study of Doppler detected planets as seems practical at this time. The results of monitoring 166 relatively nearby normal stars for five years indicates that an astonishing 1 in 4 such stars may harbor small Earth-mass planets on close orbits.
It's important to be very clear, the planets actually detected in this survey are rocky objects a few times the mass of the Earth, and they are emphatically not in the classically defined habitable zone of these stars - orbiting within 0.25 AU (a quarter of the distance of the Earth from the Sun). However, the extrapolation to true Earth-mass objects is pretty likely to be robust. It'd better be, this claim is effectively saying that there are tens of billions of such planets in our galaxy and that they outnumber giant worlds in close orbits.
This presents a big challenge to certain aspects of how we think planets form. Most current models suggest that there should in fact be a deficit in small rocky worlds in these close-in orbits, since the processes of orbital migration or runaway planet growth tend to thin this population drastically. Something is afoot - and it may also indicate that those holy grail worlds - the Earths in the habitable zone - are far more numerous than we had dared to hope. Indeed, the authors of this paper suggest that Earth-mass planets orbiting at 1AU could be more numerous than their close-orbit cousins.
Howard et al. end their paper by evaluating the implications for Kepler results. It's awfully promising - as many as 260 Earth sized (1 to 2 times Earth radius) planets with 50 day or less orbital periods should be coming our way, and who knows what in the habitable zone of Kepler stars...