I love the MSR Hubba (maximum) 3.5lbs (1.6kg).
But the recent record holder for lightest commercial 2-skin tent is the Terra Nova Laserlite (maximum) 2.75lbs (1.25kg).
How much lighter can they go? Silicon-injected nylon is very, very lightweight.
Product details — terra-nova.co.uk tents
besthike.com 