This is the first trailer for Thief, coming next year for next-gen consoles and PC.
An earlier teaser suggested that this trailer would be out on April 2, but it appears to have leaked early. The folks at publisher Square Enix seem to have a leakage problem.
UPDATE: Wow, the trailer was pulled the second I published this post. In the meantime, this Brazilian site seems to have it.
UPDATE 2: New link up top.
UPDATE 3: Square Enix has just gone ahead and released the trailer officially. It's above.