http://www.digi.com/support/kbase/kbaseresultdetl?id=2160
Direct statement from Digi
"To be clear, the inputs to the XBee module are NOT 5V tolerant and cannot accept a voltage level higher than 3.3v on any pin."
why risk it if you can avoid any problems by adding a few cents worth of resistors and diodes?
perhaps Faludi realizes the need for level shifting hence he wrote the article in his website.