AppleTV: can I watch movies from third parties?
As seen on Apple website, the supported video formats are:


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


So in order to play your DivX movies you have either to transcode them into a supported format using tools like Handbreak, or to jailbreak it so it will support more video formats.

In my humble opinion, Apple TV is the best device for streaming music, listening/watching podcasts, streaming your photo libraries from your mac to your TV, and if you are an iTunes store fanatic then buying/renting movies from the iTunes store.

If your main interest is watching divx/mkv's, then I would suggest you to look for something else.