Mark Jaquith has released a post a little while back letting all the WordPress 2.1.x users know that WordPress 2.2 means the end of the 2.1.x branch. There will continue to be security updates for 2.0.x but if you want to keep moving forward with the latest and greatest in WordPress, you need to get WordPress 2.2.
From his blog:
There will be no further releases in the 2.1.x branch. 2.2 includes security fixes that 2.1.3 lacks, so your only upgrade path is to 2.2.
The good news is that although a lot of under-the-surface code changed in 2.2, it’s not jarringly different from a usage standpoint. So if familiarity was holding you back from upgrading, it shouldn’t be.
Good to know, and really, for the most part, going to WordPress 2.2 is painless, though some people have had some small issues with Widgets, certain plugins, certain themes, and some minor interface changes that they aren’t happy with. Nothing major though.