In doing a bit of research last night I came across this white paper. The document provides developers a sample solution that demonstrates how to add new functionality into Business Portal for Microsoft Dynamics GP 10.0.
It should give you a good start on the customising of BP!
https://mbs.microsoft.com/public/insights/ReadArticle.aspx?rcpt_id=14521079&ja_id=45141
Posted by Tom Brookes