Fred, IHRA 2010 rulebook, page 107:

"WINDOW NET: If class mandates a roll cage the entry must have an SFI 27.1 safety net properly attached. ................. Net must extend forward to within two inches of the farthest back portion of the steering wheel."

The big difference is in IHRA a cage is not required until under 9.0 seconds while NHRA is more strict at under 10.0 seconds. However those who run both must be aware of the differences. Currently in LA there are two IHRA tracks and only one NHRA track.