I guess that makes sense, since the - output can't really go negative. It stands to reason that it rides on a bias voltage. You can probably tell I don't have allot of experience with differential output sensors nor do I use op-amps like this normally. Thanks for mentioning that.
True, but they are pretty cost prohibitive for my application. I found one that costs $16.09: http://www.digikey.com/product-detail/en/MPX5010GP/MPX5010GP-ND/464055But I'm still stumped about why that sip and puff iPod dock example (http://www.tmientp.com/ipoddock.htm) only has a single resistor and a 8-pin DIP IC. I e-mailed the creator about the sensor he used, and he pointed me to the uncompensated model. Curious about how to interface with this thing cheaply.