I am having this same issue.
I am using WordPress 3.5.1, Contact Form 7, and Jump Mobi Inspire theme.
I tried putting this line of code into the CSS file:
span.wpcf7-list-item { display: block; }
But, that doesn't have any effect.
I am still getting the checkboxes on one line and the corresponding labels are coming out on the next line below the checkboxes.
I want the the checkboxes and labels to be on the same line, with checkboxes first, then labels, in a block.