Comon, it makes sense for them to void the warranty and disable DRM when officially allowing jailbreak. Third party hackers with bad code could make software that actually could brick or otherwise permanently harm the hardware. Archos should not be responsible for more than how consumers use their official firmware.
With time, the open development could though provide very stable ways to run Android, Maemo Mer, Ubuntu and more. At some point it would be considered safe to install those third party installs if distributed in a way people can trust and that many people are using it without reported problems.
Demanding that Archos not void the warranty on installing this, would be like demanding that Nintendo not void the warranty when people install a modchip on their Wii, or that Apple not void the warranty when hackers brick the iphone through some bad code in jailbreak mode.
Embedded platforms are fragile, for example software should not demand from the ARM processor to constantly burst at full 800mhz mode, this would heat it up too much and could harm the hardware. The 800mhz of the processor is only supposed to be used in burst mode when launching something heavy but it should not be constantly used to convert files or something else like that for a long time. Archos carefully uses the DSP accelerator when encoding videos, but third party developers might not be careful enough about that.
Also, many consumers bought this Tablet over a year ago, or may buy it used on Ebay or something. At that point the warranty doesn't really matter anyway.