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 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.