The NVIDIA Tegra system-on-a-chip has been around since 2008 and there's now multiple versions of it out there, but its design for MIDs/PDAs/phones is vastly different from the GeForce/Quadro graphics processors. As such, the Nouveau developers aren't targeting any open-source Tegra driver.
As Lucas Stach said, "Tegras are significantly different from normal nvidia chipsets. Some knowledge can be applied, but it's likely easier to write a new driver for tegras than adapting nouveau. So there are no efforts in supporting tegra within nouveau."
Since the 2008 launch of the Tegra, Google has ported Android to Tegra, there's Tegra 2 support in Ubuntu, and there's a quad-core Tegra SoC planned for release later in the year. The Motorola Xoom is one of the modern devices taking advantage of NVIDIA's Tegra 2 with its dual-core ARMv7 Cortex-A9 processor and "GeForce ULP" graphics with just four pixel shaders and four vertex shaders.
The yet-to-be-released Tegra Stark is said to be 75 times faster than the Tegra 2, but that's not coming until 2014 at least. In the mean time there will be the Tegra Kal-El later this calendar year with its new graphics architecture that's said to be five times faster than the Tegra 2. Next year the Tegra Wayne is expected to double its performance.
While it's no surprise and makes sense the Nouveau driver isn't targeting Tegra (they have a hard time as it is in supporting all GeForce series), it doesn't help the embedded GPU situation which continues to face open-source GPU challenges. The reverse-engineered PowerVR driver is not moving along even though it's considered a Free Software Foundation "High Priority" project, there's concerns over ARM Linux GPU power management, and other vendors haven't been coming forward with open-source drivers. It's a great big mess. Some have put out partially open-source drivers, but none of them have yet been accepted into the mainline Linux kernel tree.