By default there are only a 2 options, to add it via plain xml.
before the content, and after the content.
<adminhtml_sales_order_view>
<reference name="content">
<block type="core/template" name="test-1" template="test/template.phtml" after="sales_order_edit"/>
<block type="core/template" name="test-2" template="test/template.phtml" before="sales_order_edit"/>
</reference>
</adminhtml_sales_order_view>
but those templates will be shown in all tabs.
to insert into several parts, you have to add your template like:
<adminhtml_sales_order_view>
<reference name="order_tab_info">
<block type="core/template" name="test-1" template="test/template.phtml" after="sales_order_edit"/>
<block type="core/template" name="test-2" template="test/template.phtml" before="sales_order_edit"/>
</reference>
</adminhtml_sales_order_view>
Watch out for the changed reference name.
In the corresponding template app\design\adminhtml\default\default\template\sales\order\view\tab\info.phtml
you can place your block everywhere you want
[...]
<div id="order-messages">
<?php echo $this->getChildHtml('order_messages') ?>
</div>
<?php echo $this->getChildHtml('order_info') ?>
<?php echo $this->getChildHtml('test-1') ?>
[...]
the main problem is, that the sales_order_view is kinda hard coded, so it'll keep the structure.
hope that helps.