The test kit firmware was nabbed by a hacker known as Consoledev at demonhades.org, where another member of the forums was keen to point out the leak will not lead to custom firmware creation.
"No, this does not mean CFW," he said. "But this hopefully can be useful to devs in establishing the future of CFW. This firmware is only for developers, researchers and this will only work on TEST units."
So, unless you've got a debug PS3 knocking around at home kids, the firmware won't be much use. Stick to the public 3.66 firmware that went live last month.
As mentioned above, though, it could lead to the emergence of custom firmware in the future.