I actually just spoke with a fed loan rep today, as I am in a similar situation. Finishing grad school in a week, trying to decide between PSLF while contributing to early retirement, or holding off on retirement savings to go full force at SL's. I'm choosing PSLF.
I asked her directly about the possibility of the program not existing or being changed before my 10 year/120 payment mark. She said that once you are in the PSLF program, you're in under the terms in which you signed up. So once you apply, qualify and are signed up with PSLF, they WILL honor the agreement and your loans will be forgiven. Even if the program changes in the future or upcoming elections, etc, those changes would not affect you if you're already paying under PSLF. I made sure she clarified that three times during our conversation just to make sure. ha!
Also, loans forgiven under PSLF are NOT considered taxable income...a major plus!