You can see, the public comment period has already closed; however, if the linked form still accepts comments, I see no reason not to send them…. But at this point, expressing concerns over the Keystone XL expansion will most likely be much more effective along these lines:

http://tarsandspipelines.wordpress.com/what-you-can-do/

Also, it has received practically zero public attention, but a related pipeline (a brand new route to Patoka, Illinois, and a spur under construction in Kansas destined for Cushing, Oklahoma) has sprung two “minor” leaks already. It strikes me that this is an expected result of running pipelinesâ??I calculated that over its lifetime, Keystone will leak 80,000 barrels and the proposed Keystone XL will leak 60,000.

More info:

http://andrewottoson.com/2010/07/100-gallon-leak-documented-at-keystone-pump-station-in-sd/
http://andrewottoson.com/2010/06/perspectives-on-pipeline-safety/