I have the U-Boat that Creek Co makes. It's not necessarily lightweight per se (just under 7 lbs.), but I can stuff it into a compression sack and get it down to the size of a smaller compressed sleeping bag. My whole setup comes in at just under 11 lbs (waders, fins, etc)
Barring getting a Curtis Raft (cough, cough), that's what I'm going to continue packing.
Also, I hear the Sevylor Raft is light, but super cheapo and prone to failure . I don't have one, but maybe someone else can comment.