Here is some information regarding the update two weeks ago for AT&T models. At the bottom you will find some very basic information on the update that rolled out today. Currently there is not a detailed changelog.
From the site: (regarding update 2 weeks ago)
Small update that fixes a few things in regards to the Media Link HD, which AT&T gave out to those who preordered HTC's latest and greatest. Specifically, some folks hadn't been able to connect to HTC's high-def streamer because of a software glitch. That glitch has been fixed and is now ready to download. It's a small, 269KB update, takes just a minute or so to do, and basically makes your life seem a little brighter. And fixes the Media Link HD connection.
AT&T HTC One gets fix for Media Link HD connectivity | Android Central
New update as of May 30th
Here are some basic notes regarding the new update today:
Update: . AT&T tells us this bigger update contains the new Media Link code -- and presumably something else, but we don't yet know what.