We have released a set of visio diagrams of the out of the box CRM 3.0 entities. It can be downloaded from Microsoft.com here.

 

Tip: When you create a new Entity with CRM it creates two database tables (dbo.<entityname>Base and dbo.<entityname>ExtensionBase), a basic view (dbo.<entityname>) and a Filtered View (dbo.Filtered<entityname>). You can easily point Visio at your SQL database to add these to your diagrams or using the SQL database diagram feature.