I had the same problem also.
I'm having the same trouble. It only turns up when I try to use a declarative transaction setting to something other than Required. I'm using JBoss3.0.0 and IBM DB2.
What gets me is that I'm not trying (deliberately) to use an XADataSource, so I can't understand why I'm getting an xaRes problem. I guess it's a configuration thing, but I can't figure out why it works for Required but not for RequiresNew.
Any advice? Anyone?