Since Sony, IBM and Toshiba were pushing it to be used as a general purpose CPU and Sony allowed Linux to be installed on the PS3 with full access to the Cell, it's extremely well documented. The only parts of it that weren't covered were handled when the system got hacked. IBM even had a software emulator for the Cell anyone could download for free (no source though, runs slow).
The RSX is more difficult, but still doable. Back in the early days of Linux someone discovered a bug in the hypervisor that allowed them to access the GPU directly. Within a month or so most of the RSX's major functions had been discovered. Since then, there's been multiple SDK leaks as well as a lot of people hacking away on systems with CFW and full access to the GPU.
That said, it's still a really complicated system that's going to take a while to get running solidly, and a lot longer to get running at a decent speed.