I'm uncertain if this is actually a bug, but since no one seems to have an answer we may consider that it is. For background please see the following thread:
In the end of that looong conversation you'll find my conclusion...
1) Responsive layouts setting deactivated and no custom CSS = The minimum width setting works as expected.
2) Responsive layouts setting deactivated and some custom CSS = The minimum width setting does not work as expected.
3) Responsive layouts setting activated. No matter if there is custom CSS or not = The minimum width setting does not work as expected.
There are apparently some issues with custom CSS (possibly with the some of the specific lines that I use), but more obvious is that the minimum width setting doesn't work at all in combination with responsive layouts. Would it be possible to fix this in a future release of the Suffusion theme?