I just completed a three day section (9/9/11 to 9/11/11) starting at Clingman's Dome and ending at Standing Bear Farm Hostel (just after Davenport Gap). One of the things I was testing on this hike was attempting to carry as little water as possible. There's plenty of seepage on the trail but not much of that was running enough to suit me. Here's where I found collectable water:
  • About a half a mile N of Newfound Gap there's some good spring water running of the rocks and over the trail. Good place to collect using your bottle or bladder.
  • Icewater Spring shelter (as the name suggests) has two good piped sources. The second one is N on the trail a few yards after the first one.
  • Although there is a source at Peck's Corner shelter who want's to walk 0.4 miles to get there? There's good flowing water about 0.1 down the trail towards the shelter that is collectable if you build a flume of some sort with rocks (didn't treat this although I probably should have since horses walk and crap on this part of the trail).
  • The water source at Tri-Corner Knob shelter is flowing great but horses (yes, them again) have a hitching post just above this spot so treat it.
  • A good source of water at Mt. Cammerer fire tower is day hikers.
  • There are a few trail side spring flows on the way past the Mt. Cammerer side trail but they are difficult to collect from (for me at least) and I was impatient to wait too long.
  • Just after Cosby Knob shelter (didnt' stop there due to bear activity) there's a good spring flowing on the left. You can collect directly from it easily with bottle or bladder.
  • Davenport Gap shelter had water but due to a lack of a privy and people crapping everywhere (at least it wasn't horses), I didn't touch the water there.
  • After Davenport Gap on the way N to I40, you'll cross a fast flowing creek several times. There's plenty of places to get your water filled. I didn't treat it but maybe should have. I'll post an update in a few weeks.
I think I missed one other place where I gathered water between Clingman's and Newfound where I struggled to get some water and then walked down the trail a few feet to an even better location. Just can't remember exactly where it was.

I hope this helps out. I'm trying not to carry 3L of water any more so these type of reports are very helpful to me. I understand that water availability changes due to a large number of variables so keep that in mind too.