I don't know of any design guides for new implementations of xUnit. The
JUnit design is documented in the
JUnit Cook's Tour.
NUnit was originally a JUnit port, but it was rewritten (redesigned?) to take advantage of the unique features (e.g., attributes) of .NET. I suppose you could download the source code for many/most/all of the xUnit frameworks and run some kind of UML diagram generation tool as a start to looking for the similarities and differences from a high level.
Craig
[ August 10, 2004: Message edited by: Craig Demyanovich ]