Allow audio from Mac Pro to pass through connected iPhone

this is a bizarre request, but is there an app that allows me to listen to audio from my Mac Pro via the headphones connected to my iPhone which is in turn connected to my Mac Pro?
i freelance various places and the Macs are never in a position where the iPhone headphones can reach the audio jack on the front of the machine. but i always have access to an iPhone charging cable and i always have my iPhone with me... so together the make an adequate extension... so it seems like a good idea for there to be some sort of pass-through app/feature that allows me to listen to media playing from the mac.
any suggestions?
(yes. i know i could buy an extension cable... thanks...;p and i couldnt seem to find the proper keywords to get an answer off the googles)
MacPro Quad, iPhone Mac OS X (10.6.6)