Among the items prohibited by Bill Clinton's 1994 Assault Weapons Ban were high capacity magazines. That law expired after ten years, and every shooting since then that got mass media coverage, including the one at the Aurora, Colorado, movie theater, has brought fourth calls for a return of the ban.
A ban on high capacity magazines wouldn't make any difference. The reason is that a magazine can be changed very quickly.
Here are some links to a few videos of shooters demonstrating how just how quickly. As you can see, the change takes less time than it would take to shout, "This is a gun free zone!"
Let's not forget the AK-47: here.
BTW, according to one report, James Holmes' high capacity magazine may have jammed.