Here are a few links to start working with BDC (Business Data Catalog)
Business Data Catalog Intro from the official blog of the SharePoint product group
Business Data Catalog: Overview from MSDN
Business Data Catalog samples from gotdotnet
MOSS 2007 BDC Metadata Manager by Todd Baginski (excellent work!)
and last but not least the monster MOSS 2007 resources:
Definite Resources’ List for MOSS 2007
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