  We present results from an all-sky search for unmodeled gravitational-wave
bursts in the data collected by the LIGO, GEO 600 and Virgo detectors between
November 2006 and October 2007. The search is performed by three different
analysis algorithms over the frequency band 50-6000 Hz. Data are analyzed for
times with at least two of the four LIGO-Virgo detectors in coincident
operation, with a total live time of 266 days. No events produced by the search
algorithms survive the selection cuts. We set a frequentist upper limit on the
rate of gravitational-wave bursts impinging on our network of detectors. When
combined with the previous LIGO search of the data collected between November
2005 and November 2006, the upper limit on the rate of detectable
gravitational-wave bursts in the 64-2048 Hz band is 2.0 events per year at 90%
confidence. We also present event rate versus strength exclusion plots for
several types of plausible burst waveforms. The sensitivity of the combined
search is expressed in terms of the root-sum-squared strain amplitude for a
variety of simulated waveforms and lies in the range 6x10^-22 1/sqrt(Hz) to
2x10^-20 1/sqrt(Hz). This is the first untriggered burst search to use data
from the LIGO and Virgo detectors together, and the most sensitive untriggered
burst search performed so far.
