The most substantial changes are in the HTMLWorker functionality. Some new features were added, but most of all: the code has been rewritten to make it easier to extend. This will allow us to support more tags and styles soon (in iText 5.1.0).
We also have a new core developer, Balder Van Camp, who is screening the code, optimizing performance and reducing the memory use. He has also moved the resources (such as font metrics) from the source code directories into a separate folder. More code reorganizations will follow in iText 5.1.0.
Of course, iText 5.0.6 brings plenty of fixes of bugs reported in the last three months (the previous release was on November 2nd). You can find more info in the changelog: http://tinyurl.com/itext506