There were the shortages in 2000-2001, but many still say that happened because Enron was gaming. The post Katrina shortages were from loss of infrastructure.
But the general consensus was that as the US used more and more NG to generate electricity, then domestic demand would exceed domestic supply. As you pointed out there were regions already using imported LNG, such as the NE and thru the Sempra terminal in Baja, but it was the 2000 shortages that pulled the trigger. I don't really recall exactly how many import permits were issued in 2001 and/or 2002. But it seems like Exxon, Cheniere, and Freeport.
Now it is how many export permits will be issued. Either way, a world market for natural gas gets expanded. I still remember buying into a natural gas lease in Indonesia in the mid 70s.
Not true, simple math shows this year after year. LNG imports are tracked separately from Pipeline imports. LNG imports were running about 10% of what we imported by pipeline.
U.S. Natural Gas Imports by Country
http://www.eia.gov/dnav/ng/ng_move_impc_s1_a.htm