Rich Gilcrest has written an excellent post on adding tag support to your themes, but if you’re a theme designer, I’ll also mention that some users may not upgrade their installation of WordPress right away, but would still like to use your theme. So, make sure to add a quick check to keep your theme backwards compatible.

<?php if(function_exists('the_tags')) the_tags(); ?>
<?php if(function_exists(’wp_tag_cloud’)) wp_tag_cloud(); ?>