For proprietary interfaces, I was refering to the fisrt paper (Constructing Reliable Web Applications with Atomic Actions) where they define their own toolkit (W3OTrans). In their second paper , they're using OTS which is a standard API (I'd even say that is more standard than JTA). My mistake!
As for opensource having commoditized transactions, I still think that it is true but only in the context of EJB. What we'd like to manage with JOTM is to bring the same ease of use of transactions in the context of Web applications.