Just to add to this - after checking the Scott Roberts / Hagen Green book (Designing Forms for Microsoft Office InfoPath and Forms Services 2007) the data connections in the CMCL can't be access from IP directly when designing the form - you can only select from the data connection library version. In fact, in preview, IP uses the DCL (data connection library) version - so if for security reasons you delete your DCL version after uploading to CMCL, your form won't be able to connect to the data connections after preview. But as Tom mentions - this does allow you to use a single data connection across the entire server farm - so the trade off is in the convenience factor when designing your form.
You can select an existing data connection in a DCL and then modify it to use the CMCL in the published, admin approved version of the template. The design version will continue to use the connection from the DCL.