The models used to predict the degree of warming assume that relative humidity in the atmosphere (most importantly, the upper troposphere) stays roughly constant.  So as the GHGs we add raise the temperature, more water vapour is added.  Since water vapour is a strong GHG, this amplifies the effect.

Some measurements had cast doubt on this, indicating that the relative humidity is dropping:

Theor Appl Climatol, DOI 10.1007/s00704-009-0117-x, Trends in middle- and upper-level tropospheric humidity from NCEP reanalysis data Garth Paltridge & Albert Arking & Michael Pook; Received: 21 July 2008 / Accepted: 4 February 2009 

The data come from radiosondes, which are sent up on weather balloons.  The authors themselves remark  "It is accepted that radiosonde-derived humidity data must be treated with great caution".  It certainly appeared contradicted by satellite data - see below.  Furthermore, it uses "reanalysis".  This employs data from different sources at different periods of time.  That introduces time-based biases and can appear to show trends which are not there in the individual datasets.  (See also Sherwood, S. C., R. Roca, T. M. Weckwerth and N. G. Andronova, Tropospheric water vapor, convection and climate. Reviews of Geophysics, In Press as at Jan 2010.)

But in January 2010 NOAA researchers reported there was a real and sudden drop by about 10% around the year 2000.

This may well explain the pause in the rise of temperatures in the last decade which so many denialists hail as the end of global warming.  What of the future?  It cannot keep dropping forever, so as we emit more GHGs temperatures will start to rise again.  Worse, we do not yet know what caused the drop, so it may be a temporary fluctuation.

Satellite data contradicting a drop in humidity:

Dessler, A. E., Z. Zhang, and P. Yang (2008), Water-vapor climate feedback inferred from climate fluctuations, 2003-2008 Geophys. Res. Lett., 35, L20704

Gettelman A and Fu, Q. (2008) Observed and simulated upper-tropospheric water vapor feedback J. Climate 21, 3282-3289

Buehler SA (2008) An upper tropospheric humidity data set from operational satellite microwave data J. Geophys. Res. 113, art #D14110

Brogniez H and Pierrehumbert RT (2007) Intercomparison of tropical tropospheric humidity in GCMs with AMSU-B water vapor data. Geophys. Res. Lett. 34, art #L17912

Santer BD et al. (2007) Identification of human-induced changes in atmospheric moisture content. Proc. Natl. Acad. Sci. USA 104, 15248-15253

Soden BJ, et al (2005) The radiative signature of upper tropospheric moistening Science 310, 841-844.

That last includes: "Although an international network of weather balloons has carried water vapor sensors for more than half a century, changes in instrumentation and poor calibration make such sensors unsuitable for detecting trends in upper tropospheric water vapor."