I have begun work on STM32F1 and STM32F4 support courtesy of Roger Clark's Arduino core. It's very much experimental at the moment, and we only have the STM32F4-DISCOVERY and Maple Mini boards in there (along with the JYETech DSO138 which is what prompted me to get these chips supported). I haven't yet tested under anything but 64-bit Linux, so I don't know whether the programmers (ST-Link, STM32Flash and Maple Upload) work properly in other operating systems - if people with suitable boards in their collection would like to have a play and report back that would be very handy. The biggest OS that differs, as always, is Windows.
As I port other board configurations I'm going to need people that have those boards to test them for me: I can't afford to buy all possible boards (much as I would like to), so if you have any STM32-based boards let me know and I'll focus on porting those ones first so you can play with them in UECIDE.
Also you need to be running the latest 0.10.0-RC2 version of UECIDE because there was a critical bug in the port control routines that the Maple Upload programmer relies on for resetting the board into the bootloader.
Oh, and as with all new ARM-based setups, it's using the latest Version 7 of GCC, bringing more and newer facilities to your programs that the decrepit Arduino IDE.