No, pot it up immediately - my book says recommended planting time for these is spring/summer, so perhaps you could keep the pot somewhere sheltered until early Spring.

I am confused, Giveitago. Himalayan Poppies, Mecanopsis, do noy grow from tubers, they are tap rooted perenials and I would not expect to receive one in a bare rooted state. Anyway, do as Bamboo advises. Pot it up now and and keep it in a cold greenhouse/coldframe or just beneath a glass cover. I would be interested to hear what comes up next year.
BTW, I like the name - adventurous!

I'd imagine the "tuber" is actually the tap root, Bulbaholic - that's what Eryngium planum looks like when you buy those in bags for planting. Glad you answered, wasn't sure whether it should be under glass or not.

If it was a 'normal' plant in a pot in summer then I would have said plant it in the garden. The above sounds a chancy way of buying Mecs at this time of year so I am playing safe.

I'd support the keeping it under cover this winter especially living where you do... I too am worried that a 'bulb' company would end out a tap root in a wee bag of compost at this time of year. Giveitago, out of curiosity which 'bulb' company did you buy from?

Oh and did the picture look like the images on this site? http://images.google.com/images?hl=en&client=safari&rls=en&resnum=0&q=meconopsis&um=1&ie=UTF-8&ei=uFIZS9TEHYmt4QbomZ3xAg&sa=X&oi=image_result_group&ct=title&resnum=4&ved=0CCQQsAQwAw