Check these
http://blog.customereffective.com/blog/2011/09/append-and-append-to-rights-visualized.html
http://blogs.msdn.com/b/crm/archive/2009/11/18/microsoft-dynamics-crm-append-vs-append-to-what-s-the-difference.aspx
http://community.dynamics.com/crm/b/crminogic/archive/2010/05/03/append-v-47-s-append-to.aspx
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