The environmental risks of the pipeline are quite real, make no mistake. However, the huge necessity of oil to lubricate every aspect of our economy and security is also quite real. Scary real. :shrug:
Given that oil prices are only going to keep going up and we aren't moving away from our dependence on it in any meaningful way that pipeline is going to get built eventually. When gas is $15 a gallon and food is 4 times as expensive due to shipping costs even some of the environmentalists well be calling for the pipeline. Of course, even such a pipeline won't prevent $15 gas, just delay it for a time.
Oil is a dead end road (fact) but we're still on it for the foreseeable future (also fact.). Can you imagine how much world military and political concerns will shift when we move on to some other energy source? Conflict over oil is going to get worse, probably much worse, before we figure out how to exist at viable energy consumption levels without it. But those who profit from oil are going to get fat, fat, fat off the huge profit margins as demand goes up and supply starts to taper off. Since energy (i.e. oil) is the most important single factor to modern society - thus a powerful lobbying force - I don't expect alternative energies to get much infrastructural support until the current energy suppliers are ready to control that, too. Having purchased and shelved most viable alternative energy systems already, the oil companies are poised to become energy companies - after they have squeezed every last drop of profitability out of existing oil supplies. All other consequences and considerations be damned.
It is amusing and sad that Iran, with its jet skill navy, actually poses a very real threat to American economic and security interests. We may have a huge and capable military but it depends on the oil pipe staying open. I'm sure the military has a contingency plan for oil shortages even if the civilian US doesn't - but it might not be pretty. John Q. Public will be walking to work before the military goes hungry for fuel.