We might once have called this rather uncharacteristic. Sony's eBook store is eschewing its own proprietary format in favour of adopting the open EPUB format.
The transition should be completed by the end of the year and the change will mean eBooks purchased from Sony will be useable on other readers. That's a smart move as it extends Sony's market notably beyond the user base of its own Reader devices. Plus it should make users a bit more confident that they'll remain able to use purchased eBooks, even if Sony stops making readers.
Purchases will still be DRM-protected, which is hardly surprising. Specifically, protection is provided by Adobe content Server 4, for those particularly interested in the nuances of Sony's copyright defence mechanisms. Sure, we might prefer DRM-free content, but Sony is making steps in the right directions.
The implication here is that Sony is at last learning from the mistakes it has made in the past. Minidisc, ATRAC, UMD - none of these proprietary formats did Sony any good and while the Reader line-up has been doing okay so far, it's unlikely a Sony-only eBook format will do it any favours now the competition is starting to catch up.
Smart move, Sony.