Build 12D58 comes a week after build 12D54, and like the previous build, contains no known issues or features. Build 12D54 brought significant changes to WiFi, and asked developers to focus on testing AirPlay, AirPort, Game Center, Graphics Drivers, and Safari.
9to5Mac notes that the new build again contains significant changes to WiFi, but additional changes are unknown. Registered developers can download the update on Apple's Developer Page.
This post originally referred to Build 12D58 as the eighth developer seed of OS X 10.8.3 when it was actually the seventh seed.
Article Link: Apple Seeds Build 12D58 of OS X Beta 10.8.3 to Developers