Here's an interesting heads up to running SeamTest classes inside Eclipse.
I was having some problems getting my tests, extending SeamTest, to run with the Eclipse TestNG plugin. I found this post on seamframework.org which states,
"the Eclipse Run dialog to start the TestNG test will only be viable if you have a Seam Web Project created from JBoss Tools"
I used Seam-Gen instead of JBoss Tools for my project. I ran the tests from ant and they worked.