I am trying to improve my embedded C/C++ development on ARM architecture. I have recently moved from 68K development to ARM and wanted to use some of my spare time to dig into the platform and learn the best practices especially on developing for mobile platforms.
Preferably 32bit architecture will be helpful with supporting development tools. A bit of fun during the learning process may enhance my understanding of the platform
ST Micro has a very attractively priced (and packaged too) kit for their ARM Cortex-M3 based STM32 line. MSRP runs about US$35 for the STM32-PRIMER with 128×128 color LCD, MEMS accelerometer, push button, LEDs, USB, and some spare GPIOs all in a package that includes a battery and USB to JTAG debug connection. A GCC toolchain and a commercial debugger are supposed to come with it as well. I have one on order, and will try to remember to edit this answer to include a quick review after it arrives next week sometime.
They have a new model based on an STM32 with more FLASH and RAM on chip that also has a micro-SD card connector, and a larger LCD that includes a resistive touchscreen that runs just over $100 if you can find it in stock.
Luminary Micro has a number of inexpensive demo/eval board kits as well. The people behind the eLua project (a complete Lua programming environment running in SoC and MCU scale devices) seem to like the Luminary Micro product line, but I have no personal experience with them.