Here’s a good bit of code I found in the WordPress MU forums and slightly altered for my own purposes – conditional tag based on blog ID. This will work for WordPress 3.0:

<!– SWITCH CODE BASED ON BLOG ID –>
<?php
$current_blog_id = $GLOBALS[’blog_id’];
if ($current_blog_id == 1) {
[...]

Updated: If you’ve used this tutorial to create subdomain blogs and are having trouble logging into those blogs, here’s a fix I found that worked.
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I recently spent many hours — most of it Googling and staring at error pages — trying to get the new WordPress Multisite functionality working on [...]