Hello,
rpcs3 is a multiplatform PS3 emulator and it can emulate some commercial games. It is actively developed. It could be interesting to add it as a core in Retroarch.
Just because it’s interesting doesn’t mean it’s worth the time and effort.
Unless the RPCS3 developers port it themselves I doubt we’ll see a core of it any time soon.
Personally, I’d be more interested in seeing PCSX2 (or another PS2 emulator, if there even are any) ported to Libretro.
It’s a pity but it’s not going to happen soon: http://forums.pcsx2.net/Thread-Retroarch-PCSX2-libretretro-core
Perhaps first PS2 retroarch core will be Play!: