openPMA-ng and Generation 4/5/6/7/... ;-) Archos Devices Hacking Wiki
An exploit (using the "Go Fighting Tabby" technique) has been found and published here. It works against firmware versions below 1.8.03 of both the generation 4 (604wifi) and the generation 5 (605wifi, 705wifi.) It effectively allows arbitary shell command execution as root.
A word of warning. If you mess up your bootloader or other things in your flash then the device in at least 95% of the cases will be a brick, and there is no such thing as "de-bricking". A brick is a brick is a brick. We suggest you hold your horses until we have a safe way engineered.
Notes about Opera and the exploit
Further Instructions
How-TO:Reveal Hidden Partition
How-TO:Mount root.cramfs.secure
HOW TO GET INVOLVED
Share Your Knowledge!
Suggestions for a Planned OS Structure
Info on TI's DaVinci Digital Media Chip used by ARCHOS