Timberline @ Mt Hood in Oregon
I have good news and bad news for you...
The good news: you can ski at Timberline on Mt Hood in Oregon through out most if not all of the summer.
The bad news: all Timberline has going during the summer is skiing up on a glacier (Palmer chairlift), which makes for some pretty uninteresting skiing. While Timberline's trailmap shows Palmer as a black diamond, in reality it's more of a blue or "double blue" run.
I've skied Timberline as late as the 4th of July. While its not great skiing, when it's what you have, it's what you enjoy. And I do enjoy it.
I'd rather ski the glacier at Whistler than Timberline, simply because of all the other things Whistler has to offer besides skiing. For reference, see http://www.timberlinelodge.com