It's my understanding from the last time I called about infant in arms fares (July 2011) that there's a big difference between infant in arms for B/E and economy, which is probably what they hammered you for (unfortunately). When we were looking at it, we kept my son on my wife's ticket (no status) rather than on mine (PM) because it would prevent me from being upgraded OR it would require us to pay 1/2 of the B/E fare for our son. Supposedly, Delta requires an infant in arms to pay 1/2 the lowest B/E fare, even if traveling on an award or upgrade.