Here's a quick n' dirty way to make sure any URL in a form input starts with "http://"

view plain print about
1$('form input.url').keyup(function(){
2        if (
3            ($(this).val().length >
6) && ($(this).val().substr(0,7) != 'http://')
4            || ($(this).val() == '')
5            ){
6            $(this).val('http://' + $(this).val());    
7        }
8    });

This ensures that the contents of the input will start with http://. The length > 6 is a bit of a hack, but if you delete everything in the field, or start typing over 6 chars (i.e. long enough to look for http:// at the beginning of the string), the prefix gets inserted for you. I am also loading my input with value="http://", but this could also be done w/ the function above by triggering it on page load rather than just on keyup for the specified input.