Download Squad discovered some files in the Chromium OS Git repository that suggest Acer, Dell and HP are working on computers with Google's Chrome OS.
Officially, Google has only stated that Chrome OS devices will be arriving late this fall "from select partners." So who might those partners include? If the files I spotted in the Chromium OS Git repository are any indication, Acer, Dell and HP are good bets. The overlay-x86 bits take care of configuring Chrome's hardware support during the build process -- so these would seem to indicate that Dell, Acer, and HP might be at the point where they're building the OS for specific configs.
Apart from the conspicuous files, all three have already been connected to Chrome OS in some way. Dell engineers put together a customized build for their Mini 10V netbook shortly after Google opened the Chromium OS code. As Sebastian wrote recently, HP is tinkering with Cloud Print -- which will feature prominently in Chrome OS. They're also not averse to trying different things with their netbooks -- like their heavily-customized Ubuntu remix.