What I'm doing in the same situation, is to use a BS170 (small-signal MOSFET N-channel) to pull the signal low when the Arduino pin goes high.
Arduino output -> Gate of MOSFET and Xbee Reset -> Drain of MOSFET. Source is tied to ground.
In general, I find MOSFETs easier to use for digital signals the BJT transistors like in the LadyAda tutorial.
You may also want to use a resistor (200 - 1000 Ohms) between the Arduino output pin and the MOSFET gate, to limit the current spike when switching. A MOSFET gate is like a small capacitor, only drawing current when changing state, but it can draw a lot of current for a very short while; there's a risk in very temporarily exceeding the 40 mA rating of the Arduino pins.
So, why do I thiink this is better than a solution that uses no intervening components? Two reasons:
1) With the direct pin tied, one small software bug might put the pin high and fry the Xbee. This is a very real risk. Don't ask me how I know :-)
2) The Xbee RESET signal is not specified to any particular input capacitance, so you may end up sinking more current than acceptable when driving the output pin low on the Arduino. It's a small risk, but why take it?