Speaking of titanium..... there are a few companies out there that make titanium water bottles.

They can basically do everything a steel bottle can, while only weighing about as much as a plastic bottle. Unfortunately, they're still quite expensive ($60-70 vs ~$10 for a Nalgene and ~$25 for steel).

I presently hike with an 18oz kleen kanteen and a camelback, which works pretty well for me. However, if I could find an ~18 ounce titanium bottle I wouldn't hesitate to buy it. I don't consider myself an ultralight hiker, but if I can save an ounce here and there without sacrificing anything...it can definitely start to add up.