Oceania (Australia, New Zealand & the South Pacific) - Dual passport Italian and Australian - travel to the US

I entered the US last year (2010) in mid November on my Italian passport with the WVP through transit onto Mexico where I stayed just over two months and then returned to the US for transit back to Australia in January this year (I therefore spent a total of about 2 months and 2 weeks in North America - under the 90 day limit).

My question is: As when I departed the US the final time in order to go back to Australia I didn't have to talk to any immigration officials nor hand in any tourist card or documents and so how would the US have a record of when I left the country? Also to make things more complex I self-checked myself in for my flight to Sydney (in LA airport) and in order to check in I had to use my Australian passport (for Australian visa reasons) so therefore I checked in to my flight out of the US on my Australian passport and not my Italian passport in which I had been registered under the VWP and in which I entered the US (and also mexico) in the first place. So basically would the US have a record and/or knowledge of me leaving the country in January this year? Would they know that I who entered on an Italian passport was the same person who left on an Australian?

I am just worried as when I wish to return to the US for tourist purposes in the future if they will think I never left the country and that that I stayed over my 90-day period in North America.

First off the 90 day limit applies to the US not to North America.

Secondly, do you still have the I-94W departure card in your Italian passport? This is usually collected by airline staff at check-in. If you still have this then you will have problems attempting to enter the US again as you will appear as a visa overstayer. Follow the instructions here...

http://canberra.usembassy.gov/i94.html

First off the 90 day limit applies to the US not to North America.

Secondly, do you still have the I-94W departure card in your Italian passport? This is usually collected by airline staff at check-in. If you still have this then you will have problems attempting to enter the US again as you will appear as a visa overstayer. Follow the instructions here...

http://canberra.usembassy.gov/i94.html
The I-94W has been discontinued (http://www.cbp.gov/xp/cgov/travel/id_visa/i-94_instructions/automated_processing.xml) for those using VWP and ESTA.

Ah the 1-94 departure card was not in use/not is in use anymore for at the time I traveled and so therefore it doesn't concern my situation... my question was about if CBP would be able to tell that I as the Italian that entered the US was the same person as the Australian that boarded my plane to leave the US as the airlines inform CBP of departures etc. The current system seems to be inefficient as it doesn't work for dual citizens...