Mr Blue Sky...
I just saw that Arash Ghanaie-Sichanie (the Microsoft CRM Program Manager responsible for the Server SDK) has his own blog here - http://blogs.msdn.com/arash
In particular he has posted an article on the use of Dynamic Entities vs Stongly Typed Entities in CRM 3.0 Web Services which gives an insight into the possiblities of developing true portable CRM applications, regardless of the specific entity customisations made on a CRM Server.
Check it out here: http://blogs.msdn.com/arash/archive/2005/11/23/496058.aspx
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