The new iOS for Apple iPod, iPad and iPod Touch has just been released, and one of the updates includes a fix of the notorious "Unable to purchase" bug. It can be downloaded by all users with an iOS 5.0 or higher, and it's updates include:

"This update contains improvements and bug fixes, including:
• Improves reliability of using HDR option for photos taken using the Lock Screen shortcut
• Addresses bugs that could prevent the new iPad from switching between 2G and 3G networks
• Fixes bugs that affected AirPlay video playback in some circumstances
• Improved reliability for syncing Safari bookmarks and Reading List
• Fixes an issue where ‘Unable to purchase’ alert could be displayed after successful purchase
For information on the security content of this update, please visit this website:
http://support.apple.com/kb/HT1222"

As someone who's experienced more than one of the listed issues, I'm very glad that this new iOS build came out.

Users are now able to download iOS 5.1.1 from iTunes, or as an over-the-air update.