Eric White posted great content on how to Search External Data in SharePoint 2010 using BCS. Here is the overview of the post from his blog:
This article will examine how to use SharePoint Designer to define an external content type (ECT) and how to set up a search crawl that includes that external data. You will then see how to reduce crawl times for large amounts of data by using incremental crawls. All of this will be done using SharePoint Designer, SharePoint Central Administration and some editing of XML.
Before beginning, you should have the following:
Access to SharePoint 2010 Central Administration, including the search service and the business data connectivity service.
Access to SharePoint Designer.
A SharePoint 2010 web application with a site collection. For more information, see Create a site collection (Office SharePoint Server). You can probably use Sharepoint – 80 for these examples.
Access to Microsoft SQL Server. You can use the same instance of SQL Server that SharePoint uses.
A sample database for Microsoft SQL Server (AdventureWorks will be used in these examples. See Sample Databases for Microsoft SQL Server 2008R2 to install.)
These examples will be more meaningful if you have some general understanding of SQL queries and views. Previous experience with XML will also be helpful. Even without that experience, you should be able to successfully complete all the examples.
– Lionel Robinson, Program Manager