from MySQL.com

The Commercial License is an agreement with MySQL AB for organizations that do not want to release their application source code. Commercially licensed customers get a commercially supported product with assurances from MySQL. Commercially licensed users are also free from the requirement of making their own application open source.

There is nothing in the documentation that says you MUST buy a commercial license unless you plan on violating the GPL in some way.

So, If you want to sell a product that is based on, distributes MySQL libraries or server components and don't want to release the source code to your product then you can buy a commercial license. Also, you can buy a commercial license if you want a direct legal relationship with MySQL AB.

You can run MySQL in your company all day long just like any flavor of Linux or any other GPL'ed software package.

So Adam, or anyone else that has questions about the dual licensing model I would suggest you read http://www.mysql.com/company/legal/licensing/commercial-license.html or contact MySQL AB directly like I did when I worked on my first commercial project with the prodcut.