Microsoft has fixed the digital certificate problem and last evening the company resumed publishing new apps.
The company said ot would take a day or two for the repair to fully deploy and newly-published apps to begin appearing in Marketplace again.
Since August 3 Microsoft has been deploying the Windows Phone Dev Center, which includes a completely rebuilt backend infrastructure. Earlier this week the company discovered an issue with its new digital certificates that's caused the company to temporarily stop publishing new apps.
Microsoft said that The issue impacted only phones that upgraded to Windows Phone 7.5 from an earlier version of the operating system and only apps published in the last couple of days.