It appears that Microsoft is having some issues with a few new Windows Phone 7.5 apps, and as a result has decided to stop publishing new ones as it works on delivering a fix. According to a recent post on the Microsoft blog, there's a glitch with the digital certificates used to sign apps, meaning that some apps which were published in the last few days aren't installing properly. Microsoft also says that the problem has nothing to do with your phone, so at least you don't have to worry about that.
The blog post states that this issue only affects phones that were upgraded to Windows Phone 7.5 and not phones that came with 7.5 pre-installed, but some commenters on the blog post are saying that isn't the case. Microsoft also says that this certificate issue only affects a "small percentage of the 100,000-plus apps in Marketplace," pointing to apps like The New York Times, WhatsApp, and Translator from Bing - all three of which were updated recently - as examples.
If you run into this problem, it's recommended that you just wait until Microsoft has delivered a fix. Microsoft warns not to delete the app that's giving you grief, as you probably won't be able to download it again until a fix has been issued, and you risk losing any saved data with the app by doing that. So, just sitting tight and waiting for that fix to arrive seems to be the best thing you can do in this case.
Microsoft doesn't give a time frame for this fix, so we could be waiting a little while for it land. Things are looking hopeful though, as the company says it is "already planning to roll out a fix," so it may not be that long before it arrives. Keep it tuned here to SlashGear, as we'll let you know of any new developments with this issue.