Initially floated as an Iran-Pakistan-India pipeline, the project has been pushed as a way for Pakistan to solve decades of crippling energy shortages that have led to intermittent blackouts in major cities. India withdrew from the project in 2009, saying the prices negotiated were no longer attractive.
Pakistan still wanted and needed the gas, but the US State Department has repeatedly threatened to impose crippling economic sanctions on them for daring to do business with Iran.
The insinuation of a Chinese company into the situation changes it dramatically, and seems to be the result of the latest framework deal with Iran and a planned easing of sanctions.
As a permanent UN Security Council member, China is in a much better position to thumb their nose at idle US threats over the pipeline, and given recent diplomatic progress, it will be difficult for the US to even try to retaliate over the matter.