If your thumbnail images are large enough to do what you need:
<?php wp_get_attachment_url(get_post_thumbnail_id($post->ID)); ?>
Otherwise, you will need something like:
<?php $src = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), array(300, 300), false, ''); echo $src[0]; ?>
EDIT: The array(300, 300) bit is the size of the image you want to get. WP will replace any image that has at least the same image as you. You can also use 'thumbnail' , 'medium' , 'large' or 'full' to select one of the pre-configured sizes, as well as any name defined by add_image_size() in your template or plugins.
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