Quite easy if you're using iMovie HD...the software now includes a preset for exporting to Apple TV. Read this. More details and alternative approaches can be found in the aTV discussion forums on Apple's website.
Well, the Apple TV only supports up to 720p (not 1080i), so using the preset in iMovie with HD source material from your camcorder would export video at 1280 x 720, 24p. Of course, iMovie HD itself supports up to 1080p.