Arduino meets Android

Hi, everyone, I have one project idea and I want to know if it is feasible.
Currently, I am using LabVIEW for arduino interface to control arduino talking with one SPI chip so that it could output voltage. Now I want to move the controller from laptop to the cellphone or tablet. I want to know whether it could be achieved. And which protocol should I use, bluetooth or USB?

I want to know whether it could be achieved.

Of course. Now, whether you possess the skills to achieve it is another matter.

I have my iPhone turning LEDs off and on on my Arduino. Of course, my Arduino is wearing an ethernet shield and acting as a server, and the iPhone app is a simple browser.

Your requirements may be different. Hard to say, given how little detail you provided.

And which protocol should I use, bluetooth or USB?

Well, using the bluetooth protocol over some wires is going to prove challenging, as will using the USB protocol without wires.

Pick the means of getting the data back and forth. The protocol to use will be dictated by that choice.

All the Android smartphones and tablets I'm familiar with have WiFi, so don't rule-out that possibility. Especially if you want the Arduino to communicate reliably with an Android device wirelessly from more than a foot away. Although WiFi transmitting is usually at appreciably higher power levels compared with Bluetooth (which of course is why WiFi has a greater potential range). So if there's a lot of communication from your Arduino project noticably less battery life is possible.

In any case, there are readily available tutorials, examples, and even shields or other boards to make things quicker and easier. Of course, if you want to re-invent the wheel there's nothing to prevent you.

Finally, if you want more detailed advice you'll have to be more specific on what you want to accomplish.