[CopperB]

1) Create a new file in the skin1 directory called copybilling.js
add this code:

function InitSaveVariables_shipping(form){
s_firstname = form.s_firstname.value;
s_lastname = form.s_lastname.value; 
s_address = form.s_address.value;
s_address_2 = form.s_address_2.value;
s_city = form.s_city.value;
s_state = form.s_state.value;
_s_state = document.getElementById('_s_state').value;
s_country = form.s_country.value;
s_zipcode = form.s_zipcode.value;
}

function copybilling(form){
if (form.copyb.checked){
InitSaveVariables_shipping(form);
form.s_firstname.value = form.firstname.value;
form.s_lastname.value = form.lastname.value;
form.s_address.value = form.b_address.value;
form.s_address_2.value = form.b_address_2.value;
form.s_city.value = form.b_city.value;
form.s_country.value = form.b_country.value;
form.s_zipcode.value = form.b_zipcode.value;
change_states(document.getElementById('s_country'), 's_state', 'State/Province', '', '', '', '');
form.s_state.value = form.b_state.value;
document.getElementById('_s_state').value = document.getElementById('_b_state').value;
}

else {
   form.s_firstname.value = "";
   form.s_lastname.value = "";
   form.s_address.value = "";
   form.s_address_2.value = "";
   form.s_city.value = "";
   form.s_country.value = form.b_country.value;
   form.s_zipcode.value = "";
   change_states(document.getElementById('s_country'), 's_state', 'State/Province', '', '', '', '');
   form.s_state.value = form.b_state.value;
   document.getElementById('_s_state').value = "";
   }
}

2) in skin1/customer/home.tpl

after

<link rel="stylesheet" href="{$SkinDir}/{#CSSFile#}">

add this code

<script language=JavaScript1.3 src="{$SkinDir}/copybilling.js"></script>

3) find skin1/main/register_billing_address.tpl

After the last </TR>
add this code

{* ======== COPY BILLING TO SHIPPING MOD ======= *}
<TR>
<TD align="right"></TD>
<TD></TD>
<TD nowrap>
<input type="checkbox" name="copyb" onclick=javascript:copybilling(this.form);>Use {$lng.lbl_billing_address} as {$lng.lbl_shipping_address}


</TD>
</TR>
{* ======== COPY BILLING TO SHIPPING MOD ======= *}

I have tested in Firefox 1.o and IE 6.0 and both work

Hope this helps