DCist has a copy of the letter that NPS sent to Del. Eleanor Holmes Norton on the status of the Rock Creek Park Trail project.
In it they state that right now they're waiting on the Federal Highway Administration to approve the Environmental Assessment. Now, why the FHWA has to approve the EA for a trail in a National Park is above my paygrade, but there you have it. When they might approve it is unknown - the "near future" they said - but "Construction could begin on the project as early as Fiscal Year 2015."
There is a lot more at DCist for those looking for more history on this issue.