So at Maker Faire, the Silicon Valley Forth Interest Group was advertising a Forth implementation for Arduino. I didn't actually talk to them (I had my wife with me, and she has limits!), but I remembered to look them up when I got home.
I believe they are talking about ceForth 328
This is apparently an alternative to amforth, and is a "teaser" version of Forth that will run (with some limitations) as an Arduino sketch, so that you can try it out. There is also 328eForth, which replaces both sketch and bootloader (requires a programmer) that is more full-featured and will write new Forth words to flash.
(Annoying musical website. And I can't figure out exactly when you're supposed to buy the implementation, and when you're allowed to use the free downloadable version. But it sounds promising for people who'd like to experiment with Forth.)