Designing with relative breakpoints (no pixels widths!) Crazy, but the thinking is sound: 7 Habits of Highly Effective Media Queries
The next version of Bootstrap is switching to Em-defined breakpoint widths. http://v4-alpha.getbootstrap.com/layout/overview/#responsive-breakpoints
Update: The Bootstrap team reversed course, v4 will use px breakpoints. Discussions on GitHub: #17366, #17403 (and way back in 2012: #1943)
While I suspect pixels will win out, the debate goes on. PX, EM or REM Media Queries?
Tech, noted 