OK, so it may not surprise you that I'm going to have to qualify the headline a bit lower down, but the point itself remains.
Why does anyone care about this? Answer, of course, that one of the std.septic arguments is "there is no point in worrying about human emissions of CO2, because water vapour is the dominant GHG". This argument is nonsense (which is why there is no k) but if you don't know the science thats no obvious. So...
Lets start by looking at what fraction of the current GHE *is* caused by water vapour. Its not terribly easy to find these estimates, mostly because scientifically its not an interesting question (see below). The ones I've found I've collected onto the wiki page Greenhouse effect; refs to all of this are available there. Probably the best one is IPCC '90 (first report; sadly not online) which estimates 60-70%. I presume thats a global value. Locally, instantaeously, it would vary wildly according to local conditions. If you start omitting various gases, you can push the numbers up high: Soon and Baliunas quote 88%, considering only WV and CO2. Lindzen quotes 98% (Even if all other greenhouse gases (such as carbon dioxide and methane) were to disappear, we would still be left with over 98 percent of the current greenhouse effect). I don't think thats plausible (incidentally I seem to have misrepresented it on the wiki page... oops). He doesn't quote a source for this value and may well have made it up. But apart from providing word-bites for skeptics to take out of context, there doesn't seem to be much point in these numbers. Because the main point is...
Water vapour is a "reactive" GHG with a short atmospheric lifetime of about 1 week. If you pump out a whole load of extra water vapour it won't stay in the atmosphere; it would condense as rain/snow and we'd be back to where we started. If you sucked the atmosphere dry of moisture, more would evaporate from the oceans. The balance is dynamic of course: humidity of the air varies by place and time, but its a stable balance.
In contrast, CO2 has a long lifetime (actually calculating a single "lifetime" for it doesn't work; but a given CO2 pulse such as we're supplying now will hang around for.. ohh... a century or more). It doesn't rain out (amusing factoid: the surface temperature of the deep interior Antarctica in winter can be colder than the freezing point of CO2; but this doesn't lead to CO2 snow (sadly, it would be fun) because the freezing point is lower because of the lower pressure because its higher up). So if you put in extra CO2 the climate warms a bit; because of this move WV evaporates (it doesn't have to, but just about all models show that the relative humidity tends to be about constant; so if you heat the atmos that means that the absolute humidity will increase). This in turn warms the atmosphere warms up a bit more; so more water gets evaporates. This is a positive feedback but a limited one: the increments (if you think of it that way) get smaller not larger so there is no runaway GH effect.
So: adding CO2 to the atmosphere warms it a bit and ends up with more WV. Adding WV does nothing much and the atmos returns to equilibrium. This is why WV is not the *dominant* GHG; its more like a submissive GHG :-)
[Update: 2005/02/09: http://www.radix.net/~bobg/climate/halpern.trap.html is worth reading]