A good link to read over:
http://allthatjs.com/2012/07/17/api-access-to-sharepoint-external-list/
The meat and potatoes of this post is this:
External list CRUD Operations
| SharePoint API | Read | Create/Update/Delete |
| ListData.svc (REST) | Not supported | Not supported |
| Web services (SOAP) | Supported (GetListItems) | Not supported |
| Client side object model | Supported | Supported |
Cheers,
Matthew