I picked up a pair of Vibram Five Fingers just before my recent GNP trip to wear as water crossing sandals. They were 50% off otherwise I probably never would have purchased them.
They are pretty light (not as light as Dorf's Croc's though), and once you get them on they are pretty comfortable as well (they take some extra time to get on, some of my toes just won't go in the right spot). I wore them on numerous water crossing and around the campsites. By then end of our trip, I really started to like them, and would recommend them to someone looking for a lightweight alternative to sandals.
The model I have is the 'Flow', which has a neoprene boot (it kept debris out and kept my feet warm for a few extra seconds when crossing streams)
Here they are on the shores of Cosley Lake...
PLUS, as an added bonus when stepping in mud, you leave a footprint that resembles a Sasquatch...