I've had the same problem for several days after upgrading to iOS 7, in some cases I was able to download a few apps, but when trying to download a few specific apps, I kept getting the message 'Unable to Purchase [program] could not be purchased at this time. Please try again later.'.
I tried to log out and back into App Store, I restarted my iPhone, I changed my settings to download updates in he background, checked my credit card details etc, but nothing worked. I started noticing today that the apps had an icon showing that they had already been downloaded but were not on my phone (cloud with a down arrow), but no matter what - I'd click the button to download, would be prompted for my password, waited a few seconds, and then I'd get the message again - 'unable to purchase'.
I got a tip from a friend to change the language on my phone (I'm in Norway, using the Norwegian App Store, but phone settings set to English (US) language). Changed the language to Norwegian, and voila - the apps downloaded fine!!
Actually ALL the apps I had been trying to download in the past few days downloaded automatically...