Yes, and I just confirmed with the Infinispan folks that the documentation is still correct. This is being addressed in Infinispan 6 with ISPN-604., since at the moment few of the underlying persistence systems for which there are cache stores have support transactions.
In the mean time, it does sound like the transaction would not be aware of any problems persisting the change in the cache store. IIUC, this wouldn't be an issue if Infinispan were configured as a data grid.
Definitely a concern for my application as well, which much like Brian's, is a typical Java EE/JBoss app that is or will be using JMS, JPA, JBPM etc in a transaction and will want to enlist Modeshape as just another EE transactional resource into that same transaction. So to hear that Modeshape 3, which now supports JTA transactions, is not providing basic all-or-nothing transactional semantics at the persistence level is a concern. (I'm using BJBJE as the cache store.) I'll go ahead and vote for ISPN-604.