Solving R_Thevenin in Circuit: Ignoring R_3?

[temaire]

When solving for R_{Thevenin} in the circuit below, why do you ignore R_3? Is it because the voltage through R_3 is zero?

[PLAIN]http://img197.imageshack.us/img197/88/thevenin.png

 

[vk6kro]

You would ignore R3 for the short circuit current, but you would need to consider it for the open circuit voltage.

It doesn't draw any current if it is short circuited, because the short circuit is assumed to have zero resistance.
So compared with zero ohms, whatever resistance R3 has is infinitely large and the current it takes is infinitely small, or zero.