Background
In his book An Inconvenient Truth, Al Gore states (on page 192) that seasonal meltwater in Greenland is "believed to keep sinking all the way down to the bottom, cutting deep crevasses and vertical tunnels that scientists call 'moulins'," such that "when the water reaches the bottom of the ice, it lubricates the surface of the bedrock and destabilizes the ice mass." Likewise, in their book Dire Predictions: Understanding Global Warming, Michael Mann & Lee Kump write (on page 98) that this lubrication allows "large pieces of ice to slide quickly into the ocean," which they say "could lead to a far more rapid disintegration of the ice sheets than predicted by any current models."

What was done
In a study designed to investigate this important subject, van de Wal et al. "present ice velocity measurements from the major ablation area along the western margin of the ice sheet."

What was learned
"Within days," as the Dutch researchers describe it, "ice velocity reacts to increased meltwater production and increases by a factor of 4," which response is even stronger and faster than previously reported. However, as they quickly add, "over a longer period of 17 years, annual ice velocities have decreased slightly," i.e., by about 10%.

What it means
In what amounts to a major rebuff of the claims of Gore and Mann & Kump, the seven scientists say that "in earlier work (Lemke et al., 2007; Zwally et al., 2002), it has been suggested that the interaction between meltwater production and ice velocity provides a positive feedback, leading to a more rapid and stronger response of the ice sheet to climate warming than hitherto assumed," but that their results "are not quite in line with this view." In fact, they say that their observations suggest that "the englacial hydraulic system adjusts constantly to the variable meltwater input, which results in a more or less constant ice flux over the years," such that the phenomenon "may have only a limited effect on the response of the ice sheet to climate warming over the next decades," with their data suggesting that that "limited effect" might actually be to slow rather than hasten ice flow to the sea.

References
Lemke et al., in Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Soloman et al., Eds. (Cambridge University Press, Cambridge, 2007), pp. 337-383.

Zwally, H.J., Abdalati, W., Herring, T., Larson, K., Saba, J. and Steffen, K. 2002. Surface melt-induced acceleration of Greenland Ice-Sheet flow. Science 297: 218-222.