2.6.31-17 was just released as an update (in 'proposed' on 12/10/2009, in 'updates' on 1/6/2010) but 2.6.31-18 is already knocking at the door. It just got into 'proposed'.
The full list of 2.6.31.18 changes is listed on the linux package page. There is a very long changelog, this kernel version cumulates changes from upstream versions 2.6.31.7, 8 and 9.
What stands out is the amount of changes related to ext4. Other than that, the fixes are spread out across all areas of the kernel.
Comments
This has broken my boot process.
" Waiting for root file system... Gave up waiting for root device. Common problems: "
messages are shown after my GRUB selection. Then, busybox and command line (initramfs) appears.