The Cell processor was originally designed from the ground up as a computer processor and not specifically for gaming. The Cell processor was created in a way to allow the running of muliple operating systems concurently using the different cells.
The lowest Level Cell kernel, alled Level 0, is a proprietary layer implemented mainly for security reasons, Kutaragi says -- PS3 game developers will no longer be able to directly control the hardware.
A Level 1 OS will include device drivers and a Real-Time Kernel and scheduler, and will provide all the APIs needed by real-time game developers.
A Level 2 OS(also called a guest OS Layer) might be Linux, a PC OS(Windows/OS X), or a Playstation OS, depending on how the user wishes to apply the device. Sony and its ISV (independent software vendor) and gaming content partners will sell guest OS's on interchangeable, removable hard drives pre-installed with task-based operating systems of various kinds.