If you've been following the Play Store-related news cycle recently, you are probably well aware that Google is on the verge of releasing gift cards for use on Play Store's digital goods. The Redeem functionality, along with a new Wishlist, came with the new Play Store APK v3.8.15 that started rolling out yesterday but, unfortunately, the UI does not expose it yet.
There is no web interface for redemption either, but after digging through the APK, I discovered that it was set to intercept calls to http://play.google.com/redeem. So, while you can't pop up the Redeem Activity in the app directly without hacking, removing the stock app, and installing the re-signed hacked version, there is a much easier way.

- Google Play Redeem Page Now Live In The UK, Gift Cards May Not Be Far Behind
- Elusive Google Support Page Confirms Play Store Gift Cards Will Be US-Only At Launch, Reveals Available Denominations
- 'Redeem' Option Now Shows Up In The UK Mobile Play Store, Gift Cards Likely Coming Soon
- Launch Of Play Gift Cards In UK Imminent? Google Updates Help Files With UK-Specific Information
How To: Redeem Google Play Gift Cards Using The Device Play Store App was written by the awesome team at Android Police.