More and more organizations feel the need to unlock their business data to the information workers either in the Microsoft Office or the SharePoint environment. Office Business Applications are excellent demonstrations of this type of solution. Within the SharePoint environment you can use the Business Data Catalog to connect easily to all of this data and let the users work with it with the out-of-the-box Features such as the Business Data Web Parts and for example the Business Data field type. But the BDC also has a very rich run-time object model that you can program against as a developer. Learning this object model is very important when you want to expose the business data in a custom manner to the users. One of the possible solution components is the creation of a custom field type. At U2U we have created a 2 day development course dedicated to integrating Microsoft Dynamics CRM 4.0 and WSS 3.0/MOSS 2007. In it, you'll learn plenty of things, including the creation of custom field types for SharePoint.
We have packaged that part in a walkthrough that is published as an article on our site. You can read it here and the sample can be downloaded here. It is a nice illustration of the work you can do with the BDC and it is easily re-usable when you want to hook up with other LOB-systems such as SAP or whatever.
Here is a screenshot with the end-result: