Before you get super excited, read the entire post. A spanish hacker has released a video showing CFW 3.66 running on his SLIM PS3:

Now at the moment this is pretty useless (no ABILITY to play homebrew and backup support), and you WILL loose your pre 3.6x jailbreak (unless you downgrade via infectus). It is still a work in progress, but its a good start.

Precautions and Features::

Only users in 3.55 (no matter if CFW or OFW)
No turning back, pass over to 3.66 with all that that entails. Only you can downgrade to INFECTUS (dospiedras method).
No syscalls / payloads … so that the burden of copies is disabled. For what is agreed is a firmware guy … GeoHot.
The installed on 3.55 homebrew will stop working.
No package installer (for now).
Full access to PSN.
Works on both Fat and Slim (as shown in the video), no tricks hardware and could suggest a user in the comments.

The PUP for this is available to download but we will not provide it because this is a big work in progress, we highly recommend you NOT install it and loose your current jailbreak. If your feeling risky, you can find download at source or youtube video description.

[VIA Elotrolado and Tekno]

UPDATE 1:

Thanks to PS3CRUNCH for bringing this to light, they report according to a REBUG author to Cyberskunk:

DO NOT INSTALL THIS.

After ripping this PUP apart it appears the rundown will be VERY VERY SHORT.

This is RETAIL OFW 3.66 with just a ‘few’ of the files from DEBUG firmware.

vsh.self
index.dat
version.txt

ALL OTHER FILES ARE RETAIL INCLUDING COREOS