It is a bug fix release for the 2.2 series. Just released this morning.
There is a long list of bugs fixed.
Some highlights are shown below:
# Atom feed validation fixes
# XML-RPC fixes
# Widget backward compatibility fixes
# Widget layout fixes for IE7
# Page and Text Widget improvements
And WordPress 2.2.1 is not just a bug fix release. It improve the security protect with following items.
* Remote shell injection in PHPMailer
* Remote SQL injection in XML-RPC Discovered by Alexander Concha.
* Unescaped attribute in default theme
The Upgrade steps are same as before. And I may make another post about it.