In a bizarre twist since Palm split into two separate companies, PalmOne in control of the hardware and PalmSource owning the PalmOS and software, there is a story in News.com today stating that PalmOne are looking to move to support different OSes on the Palm devices.
Quite how this will leave PalmSource is uncertain, but I am sure PalmSource are not too happy about this, since PalmOne is their main customer for the PalmOS operating system.
Other mentioned OSes are a modified version of Linux and/or Windows (PocketPC?)