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I have developed an Android app which talks to an Arduino Mega through USB.
The C code is 1.8kb and will fit on an ATTiny85 chip in order to shrink the circuit to almost nothing. The problem is...
How do I introduce USB to the circuit so I can still talk to it with the Android app?
Take a look at V-USB. I've used it with ATtiny85. It's essentially a firmware-only USB implementation for AVRs that don't have "built-in" USB.
The V-USB site also has a decent list of example projects you could use as a starting point. Not sure about how/if you can interface with android or not.
One thing to be aware of is their licensing model. You have to pick either GPL or commercial license. If you don't want to bother with adhering to GPL their commercial option has a "hobby" license for $9.90.
There are some USB solutions for AtTTiny85 available but all of them are just clever hacks. In my opinion proper USB connection can be achieved using only hardware-USB devices. You should look for devices like ATMega8u2. Chip is much bigger but software like LUFA (http://code.google.com/p/micropendous/wiki/ADK) will make a relatively stable and easy solution for you. Especially if you are rather Arduino guy than engineer.
