I didn't notice the attachment at first, and searched online for the datasheet.

Your's was from 2008, and I found this one at National from April, 2009.
There are considerable differences in the addressing scheme between the two.

And the big question is ... are you using the MSOP-8 package?

See section 1.7.4 I2C Slave (Hardware) Address
http://www.national.com/ds/DC/ADC121C021.pdf

It shows that the addresses should be between A0 and A6, depending on PIN configurations.
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