  Weak gravitational lensing from large-scale structure enhances and reduces
the fluxes from extragalactic point sources with an rms amplitude of order 15%.
In cosmic microwave background (CMB) experiments, sources exceeding some flux
threshold are removed, which means that lensing will modulate the brightness
map of the remaining unresolved sources. Since this mean brightness is of order
100 micro-Kelvin at 30 GHz for a reasonable flux cut, one might be concerned
that this modulation could cause substantial problems for future CMB
experiments. We present a detailed calculation of this effect and, fortunately,
find that its power spectrum is always smaller than the normal point source
power spectrum. Thus although this effect should be taken into account when
analysing future high-precision CMB measurements, it will not substantially
reduce the accuracy with which cosmological parameters can be measured.
