Advice needed for choosing a microcontroller interfacing with Android

I would like to make a board on which the microcontroller reads the voltage output from a sensor, and sends the data to an Android phone via USB interface. The board should be powered by the Android, so I guess Android needs to be the host and the board to be the device. Now I have some problem choosing the right microcontroller meeting those requirements:

1. Has USB capability (preferably not requiring a separate USB IC)
2. Small form factor and low cost
3. Needs an ADC pin (reading the sensor output) but excessive IO pins are not needed

Personally I have some experience with ST32M (with GCC toolchain only) and MSP430, and Arduino as well. I find board like IOIO interesting, but not sure if it is what I need (since I wish I could build my own board in the end). Can anyone suggest some IC (and/or the corresponding eval board) that is easy to get started but also good for final custom implementation?

Cortex M0+ should do the job well.
Take a look at ST / Freescale / NXP.

It would be far easier to use bluetooth to transfer your data to the Android; you could still power your board from the Android USB, provided it supports USB on the go. Alternately, some phones (specially the Samsung higher end ones) can be forced to re-configure their USB data pins to serial Tx, Rx by using a specific resistor value between pins 4 and 5 of the USB (phone end). Am not sure how to grab that data within Android, though, which is why I'd still go with the bluetooth solution.

Regards,

Anand