What's going to change in the kernel in Natty?

The short answer is: Lots.

Official:

As a kernel is released, though not before, all of the changes are written up in the Changelog: http://www.kernel.org/pub/linux/kernel/v2.6/

You'll notice they are published for each minor release. They contain absolutely everything, and are almost impossible to read, if you're not into all things kernel. :-)

Not official:

I usually read The H's summary of changes to the kernel. They've summarised all of the changes that you'll see in Natty quite well. Be warned, there are multiple pages behind each of the links:

• 2.6.36

  • Coming in 2.6.36 (Part 1) - Graphics

  • Coming in 2.6.36 (Part 2)

  • Coming in 2.6.36 (Part 3) - Architecture & Infrastructure

  • Coming in 2.6.36 (Part 4) - Drivers

• 2.6.37

  • Coming in 2.6.37 (Part 1) - Graphics

  • Coming in 2.6.37 (Part 2) - File systems

  • Coming in 2.6.37 (Part 3) - Network and storage hardware

  • Coming in 2.6.37 (Part 4) – Architecture and infrastructure code

  • Coming in 2.6.37 (Part 5) - Drivers

• 2.6.38

  • Coming in 2.6.38 (Part 1) – Graphics

  • Coming in 2.6.38 (Part 2) – File systems

  • Coming in 2.6.38 (Part 3) – Network drivers and infrastructure

  • Coming in 2.6.38 (Part 4) - Storage

and everything else as it's published

A (comparatively minimalistic) summary of the changes regarding Ubuntu in particular can be found on the Wiki pages for the pre-releases:

• Natty Narwhal Alpha 1

• Natty Narwhal Alpha 2

• Natty Narwhal Alpha 3