Linux For Tegra 15 provides preliminary U-Boot support, Vetana kernel support for U-Boot, and a new source-sync script to sync the kernel and U-Boot source-code. NVIDIA developers have also been playing around with a hardfp (ARM hard floating-point) version of the NVIDIA Linux drivers, but those are still in an alpha state. L4T 15 hardfp should be properly released "shortly", according to NVIDIA's web-site.
Linux For Tegra 15 is targeted for the NVIDIA Tegra 3 "Cardhu" and the NVIDIA Tegra 2 "Harmony" and "Ventana" flavors. Howerver, the NVIDIA Harmony device support is being deprecated and will be moving to a community-supported stack.
Linux For Tegra 15 is using the Linux 3.1.10 kernel, supports OpenGL ES 1.1/2.0, EGL 1.4, RandR 1.3, and X.Org Server ABIs 5/6/7/8/10/11/12.
The same file-system for L4T 15 is still (sadly) based upon Ubuntu 11.04 packages and not the latest Ubuntu 12.04 LTS release.
More information on the NVIDIA Tegra For Linux 15 release is available from the NVIDIA developer web-site. For the past number of weeks I've been running a NVIDIA Tegra 3 Cardhu development tablet and totally loving it -- excellent ARMv7 performance, great tablet when it comes to the hardware, etc. More Linux benchmarks of the NVIDIA Tegra 3 will be coming shortly from L4T 15.