I don't have much expierence with testing Seam but I did study it a bit.
I believe that TestNG is the best option to use, because Seam has native support for it. With TestNG and Seam you can use the ejb3 microcontainer to simulate the ejb environment which has the advantage that you can test almost every part of the application.
Christian - thanks for the link. we use DBUnit and this was an interesting read.
I have just started to experiment with a new (at least to me) type of testing framework called FIT. Basically, inputs/outputs are defined in a table structure. A "Fixture" needs to be written for each page. The Fixture is then used within a FIT table to execute tests. Here are the links I have been reading/using to setup example acceptance tests with the framework:
Blog on the framework
DoFixture Extension to Framework
Browser Control testing (using the Remote Control package)
Not everyone will like this type of testing and it doesn't replace good unit tests, but for end-end testing it might be useful to a couple of people.