CSS
.fixed-fluid {
margin-left: 240px;
}
.fluid-fixed {
margin-right: 240px;
margin-left:auto !important;
}
.fixed-fixed {
margin: 0 240px;
}
Demo: http://jsfiddle.net/andresilich/6vPqA/3/show/ Edit: http://jsfiddle.net/andresilich/6vPqA/3/
Another user asked about if this method can be adapted to the latest version of the bootstrap (v2.0 at the time of writing) so here is a demo that uses it:
http://jsfiddle.net/andresilich/6vPqA/13/