Depends on what your final goal is.
If you want to focus on the app, then
I think the way to go for this is to skip the DIY arduino part and get a elm327 bluetooth interface.http://www.ebay.com/sch/i.html?_sop=15&_sacat=See-All-Categories&_from=R40&_nkw=bluetooth+obd2&rt=nc&LH_BIN=1
Then you can focus on the app rather than all the other pieces involved with trying to communicate with
the OBDII interface with varies by car manufacturer not only at the electrical interface but also in the
in the message protocols.
Bluetooth is nice because you can then talk to the OBDII from either a PC/laptop or an android device.
The elm327 type interfaces make it much simpler because they handle much of the low level differences.
Here is a project for a GM car that talks directly to the OBDII port.
It does not use Arduino, but you can get an idea of what it is like to talk directly to the OBDII interface:http://www.nerdkits.com/videos/obdii/