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Some more points:

PIC chips are popular partly because they are available with
interesting peripherals (USB, PWM, Ethernet MAC & PHY,  etc) at low
pin counts (e.g. 28 pins) with DIP and larger SMT packages (SOP
instead of QFN or QFP), so they're easier to prototype than ARM chips.
 The PIC32 chips use a MIPS32 core.  There are instructions on the
internet for building gcc for PIC24/dsPIC, but I haven't tried them.
Max memory seems to be 32K RAM, 512K flash, with no external buses, so
not a good choice for Lua.

AVR8 does go up to 32K RAM, 384K flash (ATxmega384, ~$10 in 100
units), but I believe it's just sampling now, and for that price
you're better off with 32-bits.

ARM MCUs with >= 64K RAM and 256K flash are common, and a number have
96K or more, including the already mentioned AT91SAM7X512, the NXP
LPC238x, the NXP LPC24xx, and the ST STR9xx.  The LPC2478, for
example, has 98K SRAM, 512K flash, external memory interface, LCD
controller, Ethernet MAC, USB OTG,  2 CAN, 4 UARTS, and more for about
$12 (100's).

The closest replacement to a dsPIC would be TI's TMS320F28xx series,
which go up to 68K SRAM plus 1M external SRAM, so they should be able
to run Lua.  TI does have some affordable ($49->$69) SIM stick
modules.  TI's assemblers & compilers are available on LInux for free
for non-commercial use, but if you want an IDE or to sell
commercially, you have to pay for Code Composer ($500 to $3600).

STM32 has at least one quirk: you can't use the USB and CAN ports at
the same time, which doesn't work for me.

AVR32 doesn't interest me - since I'm an automation guy, I like MCUs
and DSPs with CAN, PWM outputs, and quadrature encoder inputs.

BTW, the BASIC Stamp isn't open source software or hardware, and
probably never will be, so its "lessons learned" aren't the same as
the Arduino.

It shouldn't be too hard to design an "eLua Stamp" using a MCU (or SoC
if you prefer) - in fact, there's no reason why there shouldn't be
several designs.  For a digital guy, I'd say the hardest part is
figuring out the voltage regulators.  My guess is that Bogdan & co
will get a working design finished first.

Finally, before I get back to playing with servo drives, I'll say that
the work the eLua team (and Ralph with pbLua) has done looks very
impressive -- it's certainly not easy to find time to do this work
after doing a regular job.