what video format does the ipad accept?

The iPad supports .mov, .m4v, and .mp4. There are apps available that can support other modes but those are the basic iPad modes supported.

read the specs:
http://www.apple.com/ipad/specs/

TV and video
Support for 1024 by 768 pixels with Dock Connector to VGA Adapter; 576p and 480p with Apple Component AV Cable; 576i and 480i with Apple Composite AV Cable

H.264 video up to 720p, 30 frames per second, Main Profile level 3.1 with AAC-LC audio up to 160 Kbps per channel, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats; MPEG-4 video, up to 2.5 Mbps, 640 by 480 pixels, 30 frames per second, Simple Profile with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats; Motion JPEG (M-JPEG) up to 35 Mbps, 1280 by 720 pixels, 30 frames per second, audio in ulaw, PCM stereo audio in .avi file format

There's an App called 'AirVideo' that can stream pretty much any format to your iPad from your Mac or PC. There's also a free App called VLC that also lets you watch pretty much any format movie.

There's also an app called StreamToMe which streams not only video, but audio as well. I bought that one over AirVideo because of the audio streaming.