Haha, weight does matter if you're carrying a lot of other stuff.
But uh, we're talking about bringing one other lens (2 lenses) rather than just one

so yes, 600g.
You don't really need to carry a lot of 'excess' stuff. If you've ever weighed your gear on such a trip, you'll see that whatever I stated above is factual and not some flight of fantasy... :dunno:
Decent backpack for camera gear with hip/waist support strap: 1.3kg
Lightweight carbon tripod with pano ballhead (Markins Q3) : 1.25kg
Canon 40D with strap and battery : 0.85kg
Tamron 17-50 f/2.8 : 0.55kg
Sunpak mini flash : 0.2kg
Sigma 12-24mm : 0.6kg
Spare batteries, filters, cleaning kit, accessories : 0.3kg
Cold weather fleece : 0.7kg
0.5 lire of drinking water : 0.55kg
Passport, wallet, map, keys, money, sunglasses (+ case) : 0.35kg
Total weight carried : 6.65kg :bigeyes:
Sure you would want to breach the 7kg mark ? Unless you're talking about the 10-22mm, if not the 70-200mm f4 IS is more than 600g...