Ethernet Shield + Pro Mini 3.3 v

I want to take an Ethernet shield R3, and plug a proto shield on the top. The proto shield will have a Arduino Mini Pro 3.3v Atmel 328. I know I need a 5v and 3.3 v power supply. I think all I need to do is connect the D10-D13 pins to the Ethernet shield. But normally these pins would all operate on 5v, but in this setup, I've got 5v and 3.3 volts. So how should I deal with this? Any other pitfalls I should be aware of?

The reason I'm doing this is the Pro Mini is actually a panStamp - it's a Pro Mini with an RF transceiver built in.

You can send a 3.3V signal to a +5V Arduino. The 3.3V is seen as "HIGH". But you can not send a 5V signal to a 3.3V Arduino.

If you look at the schematic of the Ethernet Shield, the main chip, the W5100, is at 3.3V http://arduino.cc/en/Main/ArduinoEthernetShield The ethernet board contains logical level shifters to be able to connect it to an Arduino at +5V. But to change the ethernet board, to make it run at 3.3V... perhaps that is possible, but I doubt it.