/**
* Responsive Twitter Bootstrap fixes for < IE 8 
* Greg Guydo (www.guydoconsulting.com)
* free to use and adapt for all
**/
// Desktops
@media (min-width: 980px) {
  .lt-ie8 .visible-desktop { 
    *display: inline !important; 
    zoom: 1;
  }
}

// Tablets & small desktops only
@media (min-width: 768px) and (max-width: 979px) {
  .lt-ie8 .visible-tablet,
  .lt-ie8 .hidden-desktop { 
    *display: inline !important; 
    zoom: 1;
  }  
}

// Phones only
@media (max-width: 767px) {
  .lt-ie8 .visible-phone,
  .lt-ie8 .hidden-desktop { 
    *display: inline !important; 
    zoom: 1;  
  }
}
Alternately, if you are using LESS, the above CSS file can be written as such:

bootstrap-responsive-ie7.less

/**
* Responsive Twitter Bootstrap fixes for < IE 8 
* Greg Guydo (www.guydoconsulting.com)
* free to use and adapt for all
**/

.lt-ie8 {

  // Desktops
  @media (min-width: 980px) {
    .visible-desktop { 
      *display: inline !important; 
      zoom: 1;
    }
  }

  // Tablets & small desktops only
  @media (min-width: 768px) and (max-width: 979px) { 
    .visible-tablet, .hidden-desktop { 
      *display: inline !important; 
      zoom: 1;
    }
  } 

  // Phones only
  @media (max-width: 767px) {
    .visible-phone, .hidden-desktop { 
      *display: inline !important; 
      zoom: 1;
    }
  }

}