Neither 720p or 1080p output is supported by the iPad. The highest resolution output the iPad provides is 1024x768 which falls short of 720p.
480p, 480i, 576p and 576i are supported as video output modes via Apple's accessory cables.
UPDATE: The original iPad gets 720p output with an accessory from Apple, the Digital AV Adapter. The iPad 2 supports up to 1080p output using the same adapter.