Fred,
For the IO characteristics for each pin on the CPU, please refer to the i.MX31 Datasheet from Freescale.
Here is a direct link:
http://www.freescale.com/...t/MCIMX31.pdf?fpsp=1
Chapter 4 has the electrical characteristics. As you can see, the max on any and all pins is 3.3V. Most pins, however, operate at 1.8V so keep this in mind while designing your product. If you are using the i.MX31 module, the i.MX31 SOM-LV Hardware Specification on the LogicPD website has a pin out table that indicates which I/O are at 1.8V, 2.7V or 3.3V at power on reset.
http://www.logicpd.com/auth/downloads/i.MX31/
-K