When users started to complain of inability to download both apps and app updates, which have recently made their way to the storefront, Microsoft started investigating and discovered that it's a problem with the digital certificates, used to sign apps. Apparently, only users who have updated their handsets to Windows Phone 7.5 are affected, and those who have purchased their phone with 7.5 out-of-the-box aren't. Here's the full statement by Redmond:
We’ve investigated and determined the issue only affects phones that upgraded to Windows Phone 7.5 from an earlier version of the operating system. It does not appear to impact phones sold with Windows Phone 7.5 preinstalled.
The issue does seem to be serious, if they have decided to put new app submissions on hold in the app-hungry world that we live in nowadays. Here's for hoping they will resolve it soon... no one wants to be app-deprived!