I just take the subway with my 3 yr old. I am not patient enough to hail a cab, nor do I find the drivers' skills to be within my comfort zone. There are public transit ways into Manhattan from at least two of the major airports (EWR and JFK have the AirTrain, and all 3 - EWR, JFK, LGA - have car and shuttle bus services available) so you may not necessarily need to schlep a carseat around with you.
FWIW, I don't own a lightweight stroller, so I wear DD in my KinderPack. In August, that's still do-able, but it'll be hot & muggy.
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle