The FIA magazine (free with subscription) has two articles on margin calculations for IRS and CDS at SwapClear and ICE. Both make their approach to margin calculations more transparent to a public audience than is usually available, especially ICE. Worth a read.

CDS clearing at ICE: http://www.futuresindustry-digital.com/futuresindustry/201111#pg33

IRS clearing at LCH.Clearnet SwapClear: http://www.futuresindustry-digital.com/futuresindustry/201111#pg36

Front page below.