Component can actually handle video up to 1080i. Audio is separate. The hdmi format is an uncompressed hd transmission cable with "embedded" audio. The iPhone doesn't support hmdi as the pin out on the dock connector has video and audio separated and I am guessing not in the proper audio compressor for hdmi standards. Keep in mind the audio in the iPhone (and your iPod) is completely analog audio. Hdmi would have digital audio.
In short. With component cables you can achieve up to 1080i with analog stereo audio. Formats covered would be 480i, 480p, 720p (various frame rates) and 1080i (again with various frame rates)