So I came across this review on REI:
The one drawback is the surface texture if the titanium. If water is left in the cup for an extended period of time a smell of sour mildew develops. It seems apparent that the matte finish is a perfect surface for bacteria to take hold. The cup must be washed and dried immediately after use to avoid the build up of organic matter causing the smell. It's a bit of a hassle when out in the wilds to always maintain a spotless mug. It's also a bit expensive considering how ordinary it is. Titanium isn't the best material for cooking which limits this thing to boiling water. No lid.
I had never heard of this problem, and frankly it doesn't really strike me as much of a problem, but I was curious if other people have encountered this problem, or if anyone knows enough about titanium to support or deny the assertion that titanium is particularly prone to bacteria buildup.