OLPC OS continues to be based upon Fedora Linux and with the new 12.1.0 release it's based upon the Fedora 17 "Beefy Miracle" packages.
Key features for OLPC OS 12.1.0 include the Sugar 0.96 desktop, the GTK3+ tool-kit, support for writing to the journal at any time, text-to-speech support, activity changes, a new welcome screen, GNOME 3.4 support, disk space improvements, DisplayLink USB VGA support, and miscellaneous other improvements for these low-power low-cost laptops targeted towards students in developing countries.
OLPC OS 12.1.0 is available for the XO-1, XO-1.5, and XO-1.75 hardware. With having the ARM-based OLPC XO-1.75 laptop, benchmarks of this new release will likely come in the near future.
More details on OLPC OS 12.1.0 can be found from the Laptop.org Wiki.