Some owners of iPhone and iPad devices may experience difficulty purchasing apps and media from Apple's digital content stores because they've been experiencing issues since about 12:30pm Pacific / 3:30pm Eastern Time. According to Apple's iCloud Status Page, some folks may be unable to make purchases in the iTunes Store, Mac App Store and App Store. We'll update the post as soon as these problems have been resolved.
Update 3.15pm PT: everything is working again, according to Apple.