I am seeing the same issue with Java SE 6 update 7.
I suspect that Java Web Start has changed something about the way it handles class loading which is causing this problem with GlassFish app clients but not with "regular" Java Web Start apps.
I'm in the process of investigating how we can resolve this. In the meantime, as folks here have reported, using update 6 is a workaround...if an awkward one.
More later.
- Tim
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