Zell Liew did some research on em, px and rem units in media queries. In his article he shows that pixel units should not be used in media queries. Instead we should use em units. The main reason is that the pixel based values don't work properly in Safari. When a user increases the font size of the page, the pixel based media queries don't scale accordingly.
Here is an example media query with pixel units:
@media only screen and (min-width: 800px) { … }