Joe Brinkman's Blog has all of the details.
In addition, there are newly released version of the Blog Module and the Form and List Module.
If you have an interest in upgrading, I'd recommend waiting a week or so and then check the DotNetNuke.com forums to see if there are any major issues. If not, feel free to upgrade.
In case you haven't noticed, the 5.4.x version of DotNetNuke introduces taxonomies, a user profile page for public consumption, and private messaging. There is also a provider for the Telerik RAD editor, so Community Edition users have access to this nice tool.