I have had this issue in the past with an older MBP and a different external monitor and its very frustrating. Here is the problem... I just sold my 24" iMac because I never used the thing for anything other than web surfing. ALL of my work is done on my MBP so whenever I needed to actually do something important I had to pull it out anyways. So now that my iMac is gone, I purchased a high end monitor from Dell, the 2408WFP. So I hook everything up last night...plug in the external monitor via HDMI and it shows up as a second monitor ok, but it doesn't appear to run at full resolution. I went into the display preferences and sure enough, it was set as 1900x1200 (or something similar) but the screen did not appear right. I then tried using it as the main display, but this caused even more problems. The resolution would stay real low and the graphics look like crap, even though it says its running at 1900x1200, which its not. How do I fix this? I have to be missing something, but I don't know what exactly...any help would be greatly appreciated!