[totaltec]

I haven't tested this, but it looks like you need to change:
/common_files/customer/main/register_additional_info.tpl

 {if $v.type eq 'T'}
            <input type="text" name="additional_values[{$v.fieldid}]" id="additional_values_{$v.fieldid}" size="32" value="{$v.value|escape}" />

to something like:

{if $v.type eq 'T'}
<textarea rows="2" cols="20" name="additional_values[{$v.fieldid}]" id="additional_values_{$v.fieldid}">
{$v.value|escape}
</textarea>

I have no idea if this works, or if it has any ill effects otherwise.

Hope that helps,

-Mike