The Rx and TX pins on the XBee are 5V tolerant. Powering the XBee from 3.3V, and connecting it to 5V TX is fine.
Connecting 5V to other pins on the XBee is not, because only the RX and TX pins are 5V tolerant.
OK, I'll bite. Show me on the documentation where it says 5V tolerant.
Since you didn't answer the question, I will. The datasheets for series 1 and 2 XBee say nothing about
Din pins being 5V tolerant. These are the modules Faludi talks about in his book. Not only that, but the
very first schematic in Faludi's book, fig 1-10, is wrong. He has Rx and Tx wired backwards.
On some of the "newer" XBee modules, the Din pin is 5V tolerant. So we have a pretty poor situation
here, some are and some are not 5V tolerant. You have to be very careful what you're doing, how you're
hooking them up, and how you're mixing and matching.