Will Apple TV work if I connected it to a DVD player?
The apple TV 2 supports 480p. In my case it works through my AV/Amplifier which is connected with an HDMI cable to my TV.

The only catch would be HDCP which requires end to end protection of data. But from what I understand HDCP allows non HDCP compliant hardware to receive lower quality image using legacy connectors (RCA, S-Video ...). So it all depends on how your DVD player is connected to the TV.