I am using an Xbee pro for line passing with a 5V system. Is there a problem if I connect a 5V signal to the IO pins of the XBpro (of course Xbpro is 3.3v powered)
The maximum input voltage of any of the pins should be assumed to be VCC (which can vary between 2.8-3.4V) Higher voltage runs the risk of damaging the module and is not recommended.
Check out adafruit’s XBee adapter: http://www.adafruit.com/index.php?main_page=product_info&cPath=29&products_id=126
It’s a well-designed little module that takes care of the level matching between 3.3V and 5V systems, and also has an on-board 3.3V supply (taken from the 5V system’s Vcc). I’ve got a bunch of these and they’re very handy. Her schematics are available from there, as well, so if you don’t want to buy the adapter you can at least see how it’s done.