This may be a trivial case where user 1 (uid 1) can actually be deleted. This is the first user that you create on a fresh installation of Drupal and allows for full administration rights like a root user. But when you think about it, it is critical that this user should be prevented from being able to be deleted, otherwise you are completely locked out from doing any full administration on your site and to try and get this user back is a real pain in the ass. Just as bad is the fact you can also delete anonymous user (uid 0).
Some people have reported this and it's great to see that a decision has been reach to prevent those type of users being able to be deleted. Unfortunately, this fixed has only gone into Drupal 7 which is still in heavy development. However, someone by the name of markus_petrux has kindly developed a module for Drupal 5 and 6 called Protect Critical Users which does just that.
So be sure to install it on every Drupal instances so you can sleep well knowing that user 1 or user 0 is always safeguarded once the site is handed over to your client (or some end-user).