A Single Page Web Site is essentially a web page designed to the level of a full website, and with an extended amount of content (text and images) on it.

Instead of stretching a low amount of content across multiple pages, and ending up with an entire page like 'Contact Us' just having your address and phone number on it, you could be better to concentrate all that information, keywords, images and data into one information rich page.

This method works great with search engines, make the information easy to access for your visitors and focuses all your inbound links to the one location.

An economical method of presenting a professional image
Use your own domain name rather than online directory listings
Easier to optimise for search engines - get search results in Google
Fast turn around - quicker and easier to produce than a full website
Easier to write for - focus your efforts on real information rather than padding out a full website
Works across all browsers - easier to make standard and W3C compliant
More future-proof - will work on new technology, like iPhones :)
Can be developed into a full site at a later point, if required
Can be uploaded into your existing web hosting, if available