While searching for some STM32 related data I cam across the Cortino board by Bugblat.
Like my Olimex featured in an earlier blog post this board uses a STM32F103 based 32-bit ARM Cortex-M3 CPU. The Cortino board looks like a nicely done Arduino variant. It even includes a FTDI chip. Although Bugblat does not provide support for the Arduino IDE software their product page offers a good overview of available 3rd Party IDEs.
Like for the Olimex, it should not be too difficult to adopt the Maple IDE from Leaflabs. However the Mable IDE is now getting a bit dated. A more current Arduino 1.5.5 IDE can be created by using the instructions on Makerlab.me's web page . Note unless you are fluent in Chinese you have to use Goggle Translate. However the code is documented in English and available from Github. For those that want to learn more about the details of supporting a new board there is a good document available on Arduino.cc Arduino IDE 1.5 3rd party Hardware specification. Another alternative for Mac users is the OS X native Xcode IDE as offered by embedXcode.