This may be a bit off the original Forth topic but since you are not bound to any particular programming language I would like to take your attention to one greatly undervalued language which is Pascal. One particular development environment for the AVR which I can equally recommend for the casual user as for the professional, is AVRco by www.e-lab.de . Everything you need is nicely integrated into one toolchain, plus a huge driver library (mostly written in assembler) that gets you started quickly. The strongly typed pascal language helps you circumvent many pitfalls of the C language.
Regards,
Hans