Do you realize how little water is bound up in the ice caps? Off the top of my head, I believe that it's something like 5% of the earth's total water. The desalinization won't decrease salt levels that significantly. However, it will increase sea levels; just look at how much of Laurentia (the craton which forms most of modern day North America) was covered by shallow seas from the Late Cambrian to the Silurian. Also, I've heard the statistic quoted that 80% of the world's population lives within 50 miles of a coast. Whether that's accurate or not, I'm not at liberty to say.
However, one argument I like to bring up when discussing global warming is this.
"Alright, so what's causing global warming?"
"Humans have put too much CO2 in the atmosphere, causing a greenhouse effect, which warms the Earth."
"Where did all that CO2 in the atmosphere come from?"
"Why, from the fossil fuels we've burnt, of course!"
"And where did the fossil fuels come from?"
"They're the compacted remains of ancient plants from swamps that disappeared millions of years ago."
"And where did the plants get their carbon from?"
"Ummm, don't plants get their carbon from the CO2 in the atmosphere?"
"Where did you say we were dumping CO2 again?"
Not that I don't think that global warming won't have wide-spread effects. Heck, the opening of the drake passage, a simple channel of water, led to Antarctica's current frozen state. There's all sorts of havoc that can be played when you start messing with global currents. However, I don't think that we're going to be creating an environment where humans can't survive. Honestly, the hole in the ozone layer is a much bigger threat to human survival than global warming is.