Hmm in full manual mode the camera should fire the shutter when you press the shutter button even if its not in focus. I'm not directly aware of there being a custom mode which prevents the shutter being fired if focus is not achieved through AF (when in AF mode) however there might well be one in the custom functions menu. Check your user manual for the custom functions and see if you can find it; if not you can always try resetting the camera to its default (ie factory settings) whereby this mode should be disabled (I assume its disabled by default as I've never had it happen on neither my 400D nor 7D
I was just going on my Nikon knowledge .. either focus priority (must have focus, or no shot) and release priority (actuate the shutter no matter what). Assumed Canon's have something similar. When in manual focus, it will release no matter what.. as the OP indicated in his original post. Focus priority (Canon may call it something different) is the default in Nikons... it is just that if you are too close to the subject, the AF can't focus, and the shutter wont release.
OP... how close were you to your sandwich when the shutter would not actuate in AF.. only in Manual Focus? Keep in mind that the specs for the lens you listed show 1.6ft as closest focusing distance... closer than that, and you can't get focus..