Kim - in the past year, Ranelle & I found 4 Negotiated Rate deals involving index differentials in the past year.
All were approved.  I cut & pasted relevant sections for NNG tariff sheets (the complete sheets are in the Current & Superseded sections under NNG's Informational Postings-Tariff on the EBB)

I don't have the docket number for the first one with Midamerican Energy, 1/2000.
The others are:
RP96-272-016 8/1/2001 Dynegy marketing
RP96-272-024 1/1/2001 Oneok & Aquila
  (Tariff sheet dated 1/16/2001 - there was apparently some other change after the initial filing)
RP96-272-029 4/1/2001 MIdAmerican Energy