We're releasing iText 5.1.1 about one month after the 5.1.0 because we've introduced some functionality that was needed in XML Worker 1.0.0: for instance the introduction of high level interfaces (Indentable, Spaceable) and the introduction of a WritableDirectElement class.
We've changed the logging that was introduced in 5.1.0 so that it works the same way other logging frameworks do. We've provided a simple System.out logger: SysoLogger.
We've also released some bug fixes to solve PDF reading and parsing problems, to solve problems with list symbols, to avoid ClassCastExceptions when reading/manipulating existing OCG layers.
Please be aware of two API changes: setMoreInfo now expects a Map instead of a HashMap (this shouldn't cause any problems, but you'll need to recompile your code); the default for setUseAscender and setUseDescender in PdfPCell has changed to true (which may cause your tables to look slightly different).