If you think Microsoft had shown everything during the unveiling of Windows Phone 8, it may not be the case. WMPoweruser has shown a leaked picture of what it may seems to be a virtual Arc keyboard for Windows Phone 8.
From the picture, it shows a curved or arc keyboard for a single hand usage. It reminds me of the old style Nokia phones that I had with 3 characters on one keypad.
Too early to speculate. I do hope that if it is on WP8, it can be on WP7.8 too!