I came across a great article by Karine Bosch and Patrick Tisseghem about creating custom field types in SharePoint that exposes CRM 4.0 data via BDC. The first article includes a link to download the sample.
Here are the links:
Overview - http://www.u2u.info/Blogs/Patrick/Lists/Posts/Post.aspx?ID=1789
Detailed How To - http://www.u2u.be/Res/Article.aspx?ART=CRMSharePointCustomField
The highlight here is the ability for the user to search for a CRM record (whilst checking details on the record) to attach to a SharePoint list item.