Saw this post and thought i would add a little bit of info =)
Using an ultrasonic fogger to control the humidity in a terrarium is one of the most effective methods I have ever tried, and the most versatile. I had a system set up almost identical to this that was the envy of every plant on the planet.
First off, the easiest place to get a fogger and accessories for the system is to order them from this page:
MainlandMart.com
They have may different sizes of foggers that produce lots of different amounts of fog depending on how big your terrarium is. I use just the smallest one they have for my 20 gallon tank and I used the 5 disc fogger for my 75 gallon (fogger difuses 1500ml of water per hour!!!). The other item they sell is a little bouy that floats your device at the surface of the water to maintain the maximum effectiveness of the device. This is a NECESSITY! you dont have to refill the water resivor hardly at all and you get the most dense fog this way.
The system i built is a completely enclosed air filtration/humidity system. I really wish i could photograph it, but it's all hidden in a custom cabinet, so i hope this will give you the general idea.
It's fairly simple, but kind of big and elabrate if you are seeing it for the first time. Starting at the water tank:
10 gallon tank, filled about halfway with DISTILLED FILTERED WATER (absolute necessity!). The ultrasonic water fogger sitting in a floating bouy to maintain it at the proper level for optimum performance. the PC fan pushes air into the tank, thus pushing the fog out the pipe into the terrarium. Using a larger pipe cuts down on the pressue so you get a gentle breeze laden with dense fog rather than a wet gale. the terrarium is air tight, so the only place for the air to go is out the other side into another pipe. since this is an enclosed system, there is suction being created on that end anyway, causing the fog to roll lazily through the plants and out the other side, avoiding the mist settling more than you want. on the back end of the system, there is a fresh air intake, to regulate humidity and to refresh the air supply from time to time. all i did was take a 2" pipe and cut vertical slits about 6 inches long all the way around it and fitted a 3" pipe over top of it. this slides back and forth to regulate how much "dry" air is entering the environment to drop the humidity if necessary. The fresh air mixed with the old humid air then passes through a simple air filter chamber to remove any agents that may cause harm to the system as a whole (dust, pathogens, dog and cat hair). I used a clear acrylic box with 3 hepa filters used for furnaces cut down to size. these need to be replaced yearly as they will get clogged and the chemical scrubers will loose their effectiveness. The clean air then goes through the PC fan into the water tank and the cycle begins anew. The fan is controled by a rheostat to regulate the flow of air through the system. once again, you dont want a stiff breeze passing through your plants as that will stress them on a regular basis. I like to keep the air flowing at a speed that will pass the air through the tank in about 10 seconds. Powered by a standard PC power supply.