We are just back from WDW. It does run every night and I believe stops at each of the resorts on the Seven Seas Lagoon and Bay Lake and runs through it's program.
It is basically several barges linked together with electric light shapes on each. They also have music playing at the resort beach. First it lights up the first barge with a dragon/snake and then lights up one at a time after that until they are all lit. They all have some "movement" to them as well. Then the entire group changes to American flags with stars in between. It is really nice to see, but if you don't happen to catch it I don't feel its a big deal ... certainly nothing like Spectromagic.
I believe it is out on the lake(s) for an hour or more and can be viewed from several locations (Poly, Ft. Wilderness, MK boat docks, etc.) during its route each evening. It is free and is very nice but if you don't catch it, don't feel like you've missed something major.
If I'm not mistaken, it began at the Poly right after the MK fireworks when we were there Sat, 4/3/04.