According to a tease in iTunes:
“We have a fun surprise for you on Valentine’s Day! Double tap your screen during a video call starting Monday, February 14 for a Tango treat! Make sure to update Tango on your phone to see the surprise.
The free app lets you make video calls over 3G and Wi-Fi. Since its release last October, it has become a worthy competitor to Skype.
Not only is the app free, but the quality of the video calls made using it is actually pretty impressive.
It is worth noting that video calling over a 3G network will eat into your data allowance pretty severely; so approach Tango with some degree of caution.
Tango Video Calls is available in the App Store.