Several months ago, Delta Airlines indicated it had "no plans" to release any more updates for its Fly Delta app for Windows Phone. As it turns out, that same app just received a update in the Windows Phone Store. bringing it up to version 1.16.
The change log states the new version has updated its eBoarding Pass feature "to correctly display TSA Pre-check information". It also as a number of unnamed "bug fixes and performance improvements". Keep in mind that even with this new update, it's possible that Delta Airlines may not provide any further versions of the Windows Phone app.
Thanks to Chad for the tip!