The main block holding the 'widgets' in the customer account section is
customer_account_board, so if you want to add a block there you would have to reference that one
Build this would require you to edit
frontend/base/default/template/customer/account/dashboard.phtml to add a
getChildHtml for your section.
To not be required to edit the file you could add your block under reference name
content , under this reference all blocks will be included automatically.
<block type="your/block" name="your_block" after="customer_account_dashboard" template="your/block.phtml"/>