I had a horrible incident this Tuesday when my own personal website www.asktheadmin.com got shut down for too much traffic. Normally this would not be an issue if you have a caching plugin installed. The way PHP works (the language WordPress is programed in) it requests and creates pages on the fly. So 10,000 viewers means 10,000 pages need to be generated. Using a plugin like Total Cache greatly reduces this and takes the load off the server. So if you think you have a small blog and it is no big deal – just wait until you hit the front page of dig and get burried because your site is turning up a 404.
You can grab this plugin from here.