Got my hands on Test-Driven Development: A Practical Guide today.
From my initial shuffle through this book's chapters, it looks like a good summary for getting into TDD, refactoring, and using JUnit and the xUnit family.
Most of us are already familiar with JUnit and have tonnes of resources for TDD in Java.
For those looking for some .NET TDD articles, over at The Code Project, there is an introductory TDD in .NET article and an Advanced Unit Testing article series: Part 1, Part 2, Part 3, and Part 4.