I have been researching ultra light tents for more than 2 months now. I finally settled on the Big Agnes Seedhouse SL2 2011 version. It is a near perfect tent in terms of size, weight and price. When its packed with all extras the tent weighs only 1,5 kg. It cost about 300 US dollars and is only 40X17 cm when packed in its bag. There is plenty of room (213cm X 132 cm sleeping area) if you are traveling by yourself and a bit snug if you are two, but you can totally feel comfortable two ppl as well.
Here are some other great contenders: