  We combine new analysis of the stochastic gravitational wave background to be
expected from cosmic strings with the latest pulsar timing array (PTA) limits
to give an upper bound on the energy scale of the possible cosmic string
network, $G\mu < 1.5\times 10^{-11}$ at the 95% confidence level. We also show
bounds from LIGO and to be expected from LISA and BBO.
  Current estimates for the gravitational wave background from supermassive
black hole binaries are at the level where a PTA detection is expected. But if
PTAs do observe a background soon, it will be difficult in the short term to
distinguish black holes from cosmic strings as the source, because the spectral
indices from the two sources happen to be quite similar.
  If PTAs do not observe a background, then the limits on $G\mu$ will improve
somewhat, but a string network with $G\mu$ substantially below $10^{-11}$ will
produce gravitational waves primarily at frequencies too high for PTA
observation, so significant further progress will depend on
intermediate-frequency observatories such as LISA, DECIGO and BBO.
