So far I have an nrf51822 cheapy module. It was about £5 on ebay and has two lots of 18 anoying 1.27mm spaced DIL pins. I've read about people programming these with mbed generated code (https://mbed.org/forum/platform-39-Nordic-nRF51822-community/topic/4982/) and using the programmer built into STM boards (Sorry, Can't find link ATM) so I'm going to try that with one of the mbed demos once I get the headers figured out - both the pinouts and how the hell to connect them up, 1.27mm IDC plugs are like £7 each!
As for the ARM chip, I'm already almost certain about using Energy Micros ARM EFM32 series. They have great hobbiest support, a nice free eclipse based IDE (at least for windows) and support GCC. I haven't settled on a chip yet but I do have the Zero Gecko development board (pictured) with the Sharp memory LCD. It should be useful for its real time power use monitoring / profiling tools and for evaluating the sharp memory LCD. The Zero Gecko doesn't have native USB which I would like the option of (for firmware updating, loading configurations).
The only thing in the negatives column for Energy Micros is no mbed support. I would like to use mbed if possible. I really like the Freescale boards (its my go-to arduino replacement) and the dev boards are SO cheap so freescale are still a contender, I need to check the power use though.