The Arduino Due will have an Atmel SAM3U processor.
The Arduino team said in the announcement "We plan a final and tested release by the end of 2011".
The Atmel SAM3U processors are O.K. but there are a lot of other interesting ARM processors.
Will the Arduino ARM environment be modular enough to support other "Arduino-compatible format boards" like Maple?
This is not as easy as with AVR work-alike boards. ARM processors from different vendors may have a ARM Cortex core but the chip's peripherals are very different.
Clearly vendors of work-alike ARM boards will need to do a lot more software to use the Arduino ARM IDE.
Maple uses STM32 processors and has some very nice hardware http://leaflabs.com/store/
There are other inexpensive STM32 boards.
The $10 STM32LVDISCOVERY board is nice http://www.st.com/internet/evalboard/product/250863.jsp
. Clearly ST is subsidizing the cost of this board. Lots of websites have tutorials using this board with GNU software.
Several projects use this board http://www.xduino.com/hardware/
. I bought several here http://www.futurlec.com/ET-STM32_Stamp.shtml
for about $25 each. Fururlec doesn't have a very good rep though.