I've got a project that works by delivering current to the user's mouth. I'm currently using a 5V digital out, but it's just not the same as a 9V battery. I'm just getting into electronics, but is there some component that will increase the voltage from 5V to 9V? In the hydraulic analogy, I think this would be like a small pipe coupled to a big pipe? Not sure.
There are voltage doubler circuits. Use google to find them. You can also use one of the new voltage upconverter chips.
Or rather, seeing as how zacharyr seems fairly new to the mindboggeling world of electronics, he should use a transistor.
Pretty beginner'ish (you'll need to learn it anyways) stuff if you can live with using, say, a 9V battery.
Get a hold of a current-limiting resistor so you don't burn your precious Arduino. 370-500 Ohm should be safe.
Connect this between the desired output of the Arduino and the middle leg of your 3-legged transistor. Proceed to connect the emitter-leg of the transistor to Arduino ground (GND). To find out which legs are collector and emitter, you'll have to google or something.
Now, connect whatever stuff you want to control (within the limits of the transistor, say max 12V) to the transistor:
Hook the positive lead of the battery to a resistor, I'm guessing about 1KOhm (1000 Ohm) would be a good place to start. Connect the other end of the resistor to the transistors collector (it will be the only wire connected to this pin on the transistor), and connect the emitter-leg of the transistor to the negative lead of the battery, through whatever load you're gonna use (motor, light-bulb, loads of leds, a relay, your tounge..).
Basically, when the transistor has a small voltage applied between base(middle leg) and emitter, it allows a larger current/voltage to pass between it's collector and emitter.
If you don't get my crappy explanation, tell me and I'll draw you a diagram tomorrow. =)
Btw, why the tounge? Are you experimenting with sensory feedback stuff? =D
Your +5 volts can turn a transistor on and off, the transistor would control the +9v
If you needed more you can control a solid state relay that can control much more voltage and current.