There are two ways to hook up video to some modern monitors. The normal way is to use a PC input (VGA, DVI and sometimes HDMI). The second way treats it more like an HDTV, usually HDMI.
Depending on your choise of monitor inputs you are either putting out PC data or VIDEO data. For example, if you hook up a Cable box to an HDTV using HDMI it puts out a single video output such as 1080p. If you are putting out HDMI-Video at 1080p you are LOCKED to a constant 1920x1080 resolution. You may even have some overscanning or other scaling issues to deal with. You absolutely do NOT want to use this for a PC (though it's nice to have for a PS3 or other device as the PS3 puts out Video).
My dad can watch high-def BluRay rips (mine of course. cough.) using his laptop hooked up to his HDTV via HDMI. This is a VIDEO signal and it took a little tweaking at first to get the overscan setup (through the NVidia Control Panel). In theory I could have used the PC-VGA input that the TV had but it was limited to 1366x768 and it's a pain to remove the cable. VGA has no HDCP support meaning it can't support BluRay from a PC. (I also had to toggle the Audio to "HDMI" which turned off the laptop speakers and sent all the audio out the HDMI cable as a PCM Stereo signl.) This looked and sounded amazing though.
You need to ensure that you are using one of the PC inputs. You'll know if you get a choice of multiple screen resolutions, usually 800x600 up to 1920x1080.