There is a specific wire format for use with collections. This is discussed on Scott Hanselman’s blog here:
Another blog entry from Phil Haack talks about this here:
http://haacked.com/archive/2008/10/23/model-binding-to-a-list.aspx
Finally, a blog entry that does exactly what you want here:
http://blog.stevensanderson.com/2010/01/28/editing-a-variable-length-list-aspnet-mvc-2-style/