I recently met with several members of the dev team that developed the "Rhythm of the Business" OBA at Microsoft and asked them about their project experiences, challenges encountered, learning curve, differences between Office 2003 vs 2007 development, etc. You can see their answers and insights here:

For more info about the ROB OBA, check out the recent ROB case study available at:

http://www.microsoft.com/casestudies/casestudy.aspx?casestudyid=201168

A technical article detailing some of their work is available at: 

http://msdn2.microsoft.com/en-us/library/bb194905.aspx

- Khawar