My setup: Windows 7, nVidia G210 HDMI, Marantz SR6004, Panasonic plasma.
If the amp is put into standby and then turned back on, often the media centre will output a low resolution once everything syncs. The fix for this is to go through the display settings wizard in media centre whilst it looks wrong, correct the res there, then save it.
I don’t get this when resuming the media centre from sleep, only when it has been on and effectively disconnected by the amp being powered off.
All credit here: http://richardstk.wordpress.com/2011/02/10/incorrect-screen-resolution-when-resuming-from-sleep-with-windows-media-center/