From the WSJ:
Ratings Move Roil Bond Insurers The first potential casualty that went public was Canadian Imperial Bank of Commerce, which said it is a hedge counterparty with ACA Financial on about $3.5 billion in U.S. subprime real estate. CIBC said it could report a large fiscal first-quarter charge related to ACA Financial's rating cut and its ability to remain as a viable counterparty.
The informed speculation was that CIBC's counterparty was ACA. That was confirmed by CIBC this morning. See:
Counterparty Risk: CIBC and ACA. Others will be lining up at the confessional.