it's not at all a good idea to intermix both power supplies.

Well, I do it in most of my designs. The Cyg.. argh SILabs chips that I use for speed are only avilable in 3.3V and some of my I/O require 5V.

Intermixing is easy with chips such as

http://www.semiconductors.philips.com/pip/74LVC...245AD.html

Erik