Inov8 do a Goretex version of their Roclite 325 boot (which weighs 325 grams for a size
. That's probably about as lightweight as you'll get, I think. The trade-off is that they're not the most durable boots in the world, so won't last quite as long as bigger, heavier boots.
I recently got the non-waterproof version. I haven't hiked in them a massive amount, but I have found them extremely comfortable and pleasant to walk in. I don't really mind getting my feet wet as long as they don't get blisters (which they don't in these) and don't get cold (which they don't if I'm moving about). It's also nice that your feet dry-out quickly if there's a dry spell.