I found this table in one of the documents examined, that compares the ratings for different ISOs:
Company	Minimum Long Term Rating	Minimum Short-term rating	
CAISO	A3/A- for Market Obligations A1/P1 for Grid Management Charge	A2/P2 for Market Obligations A1/P1 for Grid Management Charge	
NEISO	"INVESTMENT GRADE" (equivalent BBB-)	"INVESTMENT GRADE" (equivalent A3)	
NYISO	BBB/Baa2	or equivalent	
PJM Interconnection	Based on PJM internal scoring system	Based on PJM internal scoring system	
Ontario IMO	BBB*	or equivalent*	

* The financial support obligation of a market participant may be reduced by a monetary value associated with a particular credit rating. AAA = 50 MM CAN$; AA = 25MM CAN$; A = 12.5 MM CAN$;and BBB = 5 MM CAN$. Interesting!

This table is current as of March 2000. Indeed, I think it might be a useful format to present the information found. Regards,
ALF