In an app I've worked on we implemented a feature allowing a user to gift a (one time) one month subscription to another user.
The app has been rejected 2 times because, accordingly to the TOS, no physical item could be exchanged with the in-app purchases.
Replying and assuring in both cases that no physical item was exchanged didn't change anything - app was still rejected, so in the end the feature has been removed from the app.
Update 2014-04-14
Just found this page, explicitly stating that
You can't gift an In-App Purchase
(expand the Types of In-App Purchases section).
That should solve any ambiguity...