I had a look around and I found the specs for your TV.
Q9 50inch (PS50Q91HDX) plasma TV - television SAMSUNG
Manual for TV http://downloadcenter.samsung.com/co...X7_Eng0321.pdf
Review for TV (Confirms 1080i capable ... This is still better than 720p
Nice TV by the way
I have read a few articles that state HDMI is a hit and miss situation with the XBOX. If you have component cables you can still set your XBOX to 1080i with none of the "handshake" issues associated with HDMI.
I cut out this info from here: http://forums.cnet.com/5208-7596_102...sageID=2585261
"The Player (in this case your Xbox), has to confirm that the Display (your TV) is HDCP-compliant. The process is called a "handshake". So the Player sends out a message to the TV saying "Hey Display, are you HDCP-compliant?". If the Display returns confirmation in time, then they "shake hands". If the TV doesn't respond quickly enough (the amount of time necessary is determined by the player), the Player will not transmit content properly to the Display; instead it displays a green screen, or blinks on and off every second, etc. I suppose your Xbox is more polite and tells you that the TV is not satisfying its needs.
The reason why component video works fine is because it has no such DRM."
I found this info as I had a HDCP link error when I tried to watch a DVD or HDDVD. I replaced the HDMI cable I got with the XBOX and replaced it with a 3rd party cable and it works fine now. (I have heard this has not worked for other people though, so an investment in component cables may be a safer option for you)