I have an Omnigraffle canvas sized at the exact size of U.S. letter format (8.5 X 11 in). I want it to print out without any margins, but can't seem to find the correct settings to do so. I have set the canvas size and page setup properly, but the page always prints out with a small margin on all four sides.
Did you set the margins to 0 under the document inspector?
Edit: never mind this step, you already had it set under Page Setup
Also, make sure to select borderless by going to Paper Handling > select Scale to fit paper size > US Letter Borderless