2.6.31-20 kernel just got released in "proposed". At first sight it's the same as 2.6.31-18. It has the same very long list of bugs fixes. What that means is that 2.6.31-19 was really ONLY a security release and it didn't get the bug fixes included in 2.6.31-18.
Keep in mind that proposed that software released in "proposed" is only for the early adopters. If you still have 2.6.31-17 and you have to upgrade, go for 2.6.31-19.
Full list of fixes is available on the 2.6.31-20 changelog page.
