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.
