I had a bike once with a front 27 and a rear 700 , it works 
To be super sure, ask these guys : http://forums.roadbikereview.com/forumdisplay.php?f=76
OK. From what I can gather, the ISO Bead seat size of a 27 x 1 1/4" tire is 630mm, and a 700 C bead seat is 622mm, which means in my limited thought processing that the 7oo C wheel is only 8mm smaller in diameter, half of that, 4mm, in actual difference from the axle to the rim itself. SO…, I am wondering if I can update my older commuter to a set of 700C wheels and rock sweet modern tires, all year round. Like studs and cross tires. Back to slicks in the spring time. I figure I can drop my brake pads down 4mm to hit the new wheels. Any thoughts? Anyone ever do this???