Apple has begun including a "unique_identifier" field in its in-app purchase receipts, reports MacRumors. The identifier may be a first step towards closing the hole that was exploited last week to allows users to make in-app purchases without actually paying any money. Although it's not identical to a devices UDID, as The Next Web points out, it could be a hash of the same or some other unique identifier.
It's not clear exactly what the intent of the identifier is yet, but the fact that it has been added just after the hack may be a sign that it used to combat it in some way. Apple said that it takes "reports of fraudulent activity very seriously and [it is] investigating," but beyond that and some other efforts to block IP addresses, it appears that the security hole still persists.