The sDom syntax is a little bit obscure at first glance. I've done it this way in order to keep it concise. Possibly the best way of thinking of it is that
I initially thought that the divs referred to existing divs, but now I understand that the datatable actually injects these divs are the given position.
Also, the full_numbers pager does not render the page numbers on the top (but it does render the first/previous/next/last controls on the top) when you request rendering it both on the top and the bottom.
Rendering elements twice (pager, info etc) is not supported in the built-in DataTables DOM handling. If you want to do this you need to either use the API, or modify the source code. The API will certainly help with the paging, but there isn't really a method to deal with the info element directly. What you could do is use the fnDrawCallback() function and update other elements on the page as you see fit.
I just want to modify the default layout a little bit (move the length dropbox to the bottom). Can someone post the positioning string for the default layout?