Where can I find pin mappings for the ATMEGA168?
Page 2 of the ATMega168 data sheet has the pin mappings for all four packages of the chip, and pages 3 and 4 give a bit more information regarding some of the basic pin functions:
Of course this can be found in the atmega168 datasheet from Atmel but sometimes it is also handy to know which Arduino pins correspond to the Atmega168 pin numbers. This reference can be found on the Arduino site.
Another very good resource for everything AVR is the AVR Freaks community.
They have a Device list (Gone, use Atmel's here) (ATMega168 is here) where you can get a quick overview of the chips features, the pin mappings, and of course the official data sheet, too.
Personally, I prefer AVR Freaks for my projects. Ironically, the default search on Atmel.com is google-wide - and directly links to AVR Freaks, too.
Be aware, shameless promotion ;)
Here is a cheat sheet with the ATmega-Arduino pin mapping on it. http://tinkerlog.com/2009/06/18/microcontroller-cheat-sheet/
You may find it at http://www.msarnoff.org/chipdb/atmega168. Very plain and simple site. They will also link you to the original ATMega PDF document if you need more than just a pinout.