First generation devices that may be impacted by the issue include: Dell Venue Pro, HTC 7 Pro (Arrive), HTC 7 Surround, HTC 7 Trophy, HTC 7 Mozart, HTC HD7, LG Optimus 7, LG Quantum, Samsung Focus and Samsung Omnia 7.
Most apps and the newer devices are unaffected by the issue, and users will be able to continue to successfully download and install apps. However, until the issue is resolved, they will not see any new apps, or updates for existing apps published to the Windows Phone Marketplace.
Here's how Microsoft describe the problem:
We’ve run into an issue with the digital certificates used to sign apps, and this is preventing some phones from installing some apps published during the last couple of days.
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.
Digital certificates are used in the app publishing process as a security tool, ensuring that a downloaded package is "valid". Without certificates it would be possible to install content from any source. While the certificate requirement does frustrate users who want a completely open phone, it is generally regarded as being to the benefit of the majority because it helps prevent the installation of "untrusted" content, which could, potentially, contain malware or unstable software. (The best solution would be to make it so users could disable certificate checks, as you'll find on other platforms - Ed)
The number of apps affected by the certificate issue is, according to Microsoft, "only a small percentage of the 100,000-plus apps in the Marketplace", with the impacted apps being those that have been added or updated recently. A number of high profile apps do fall into this category, including the New York Times, WhatsApp, and Translator from Bing.
Microsoft say they have already identified the problem and working on a fix:
We know what’s causing the issue and are already planning to roll out a fix. Until that’s finished, we’re going to temporarily stop publishing new apps in Marketplace.
Microsoft recommends that users affected by the problem, which manifests as an error message when installing or updating an app, wait for Microsoft to fix the problem. Users should avoid uninstalling apps because they may be unable to download them again until the issue is resolved.