A great introduction to OBA: what they are, why they're powerful and an introduction to the key aspects of the architecture:

http://msdn2.microsoft.com/en-us/library/bb614538.aspx
http://msdn2.microsoft.com/en-us/library/bb614541.aspx

This chapter is an excerpt from a forthcoming book on OBA:

7 Microsoft® Office Business Applications for Office SharePoint® Server 2007