Well, the RoboStix uses the AVR ATmega128, so I'd say that it's a sure bet to use it with AVR Studio.
Here's the specs from the GumstixDocsWiki:
* ATmega128 AVR processor at 16MHz
* 30 AVR GPIO lines at 5V
o 6 optionally function as 16-bit PWM outputs
o 8 optionally function as 8 or 10-bit ADC
* 2 AVR UARTs at 5V
* 2 Gumstix UARTs at 5V (STUART and FFUART, when used in conjunction with a Gumstix module only)
* Shared I2C bus for communication between the AVR and an attached Gumstix module, and control of external devices
* ISP connector for programming AVR (can also be programmed by an attached Gumstix module)
* Power Jack (5.5V+) or separate headers for logic and servo power
Try going here: Robostix
for more information, particularly the Intro and the FAQ
With 30 GPIO, I'd say it could handle 12 servos easily, even double that number.