I having the same problem, when having an aspect transaction running and creating new transactions with requiersnew.
I got the same WARN message in the log as markus.
It's only show up in some of our larger tests, and we fail to recreate it in a small isolated test.
We are trying to understand why we seeing this and how to recreate so that we later on can avoid it.
The TransactionAwareDataSourceProxy fix works for this, but we don't want to us that if there is a another way of doing this.
And btw, we are using JBoss 7.1.1 at moment.