The Cortex M3 based boards I've been playing around with are quite astounding, the MCUs they use as Systems-on-a-Chip have a very nice range of peripherals which really only leaves your imagination as a limit to what can be done. Being able to interface with off-the-shelf MMC cards for example, or talking to a simple LCD module is pretty neat.
The only major drawback is that those chips usually have very little RAM, even compared to the Newton ;) - eight to 64 kBytes are not exactly a lot. When using CoreForth as my experimental platform, it is therefore interesting to play around a bit with different memory management approaches. Forth itself usually likes to live in RAM since it is more powerful when self-modifying code can be used, but there are ways around this limitation.
When it comes to multitasking though (which is relatively trivial using a Cortex M3 since it pushes most registers onto the stack when an interrupt occurs), a traditional preemptive threading model is quite wasteful what comes to RAM as each thread needs a separate stack. But the revival of event based programming through Node.js led me to check local continuations, in other words, functions with multiple entry points. In this area, protothreads look particularly promising for an implementation in Forth!