Personally, I'm running a mineral oil computer with an attached chiller, and I'm maintaining about 55C on my GTX 470's (its a gaming rig, only started on bitcoins recently) while mining. I got into computer cooling as a hobby several years ago, and have played with liquid cooling, mineral oil, and mechanical chilling. From what I understand, every 10C over 50C you roughly halve the lifetime of the component in question, though I don't know what the normal expected life of a well cooled card is.
If you're maintaining 90F on a card while mining, I'd be astonished. I don't know how you'd manage that without some serious phase-change equipment.
I don't get how that could be true when most modern GPU's I've dealt with have idled over 60C... do you have a source for that?