I also will install the new one when I think it will need it (bought one off ebay right after I bought the car July 09).

I perused the 04 service manual set in depth after seeing the varied comments here in the forum back in august 09. Unless I am missing something the location of the filter and ductwork suggests the airflow from passenger side to center (drivers side). I have not taken out the filter yet, but its shape does limit the replacements installation. Some may force and crush the new one into the space threrby making the replacement moot but it looks simple and easy. Those who have none in place, sliding it in gently without forcing it should allow you to install it correctly (nothing bulging out and/or crushed to fit). This seems to be one of those topics that have just as many people saying it goes (airflow) from right to left, and those who say it goes left to right.

I might just take some time and go take mine out and see which way they installed the original.