Ok, so instead of futzing around with an adapter (some of which i hear don't even work right), I decided to splurge on something I should probably have bought a while ago: a usb audio interface. now this doesn't erase the fact that i've lost some faith in apple, but it does solve the loose headphone-jack issue. check it out:
http://www.amazon.com/gp/product/B000KW2YEI
sure, it's 30$, but i can say that it indeed works. i do a lot of headphone listening and it's totally worth it. plus i record into garageband from my mixer, so this works better than the line-in anyway. behringer is a pretty trusted name in audio, and if you're skeptical about this device, just read the reviews on the product page. happy listening!
(oh yeah, if you do buy this, make sure to go to System Preferences, then the Output tab, and select the USB interface for sound output. then just plug your headphones into the interface's headphone jack)