Adding !important using a Compass Mixin

You can include it inside the mixin like so:

@include border-radius(5px !important);

Compass will output the following:

-webkit-border-radius: 5px !important;
-moz-border-radius: 5px !important;
-ms-border-radius: 5px !important;
-o-border-radius: 5px !important;
border-radius: 5px !important;