Ryan’s explanation of the is_* functions in WordPress 1.3 is an excellent summary of some of the new functions in this version of WP.
In WPMU, it should be possible(I haven’t tried yet!) to do the following from your Smarty template:
{if is_search()}
    <b>The Results of Your Search:</b>
{/if}
It’ll display the message “The Results of Your Search:” when a search is performed on your site.