The warp zone was well hidden.
At the first Nintendo roundtable, Super Mario 3DS producer Yoshiaki Koizumi revealed that there was a warp zone hidden in one of the levels featured in the Super Mario 3DS demo. Both Michael "TYP" Cole and I were searching for this fabled location during the last day of E3, but were having quite a bit of trouble because the hint word "turtle" was somewhat misleading. During the last hour we actually found it. Watch the video below. TYP is playing the game in the video.
Quick Links to Super Mario 3DS impressions: