OpenWrt 12.09 has switched to using the Linux 3.3 kernel and switched to use uClibc 0.9.33.2. Additionally, the x86 OpenWrt images have finally switched from using the EXT2 file-system to now on EXT4.
Other OpenWrt 12.09 features include improved parallel building, the use of shadow passwords, firewall improvements, support for external overlay file-systems in release images, wireless driver updates, experimental support for 5/10MHz channels in ath5k/ath9k, support for 6RD, and much more.
OpenWrt 12.09 also has new targets for the Raspberry Pi (BCM2708) and RAMIPS. There's also support for some other newer router models too.
More details on the OpenWrt 12.09 release features can be found within the release announcement.