For libretro right now there's RetroArch as a multi-system front-end, but support for libretro directly within XBMC would allow the system to play various games/emulators. Among the platforms supported right now are Linux, Windows, OS X, PlayStation 3, Xbox, Gamecube, Wii, Raspberry Pi, OpenPandora, Android, BlackBerry 10, and Apple iOS. For more details on the upstream libretro / RetroArch project itself, visit Libretro.com.
The news now is that yesterday there was a submitted pull-request for merging the RetroPlayer proof of concept into XBMC's code-base. This pull request provides the XBMC libretro support for Linux and Windows platforms. More details on the RetroPlayer XBMC support can be found in that pull request or via the XBMC RetroPlayer sub-forum.