Also might be of interest: Bunnie's hack of SD cards last year http://www.bunniestudios.com/blog/?p=3554

"An Arduino, with its 8-bit 16 MHz microcontroller, will set you back around $20. A microSD card with several gigabytes of memory and a microcontroller with several times the performance could be purchased for a fraction of the price. While SD cards are admittedly I/O-limited, some clever hacking of the microcontroller in an SD card could make for a very economical and compact data logging solution for I2C or SPI-based sensors."

"The embedded microcontroller is typically a heavily modified 8051 or ARM CPU. In modern implementations, the microcontroller will approach 100 MHz performance levels, and also have several hardware accelerators on-die."

Was discussed on HN, but Algolia search looks to be down at the moment.