After upgrading to Magento v1.4.1 I was treated to the following, nasty PHP error:

Fatal error: Call to a member function toHtml()
on a non-object in * on line 529

As with most things, I’m sure others are having this problem, but fortunately the solution is easy.  Just update the file app/design/frontend/base/default/layout/page.xml by adding the tag name=”core_profiler” to the HTML element <block type=”core/profiler” output=”toHtml”/>.

The line should now look like this:
<block type=”core/profiler” output=”toHtml” name=”core_profiler”/>

Hope this helps you out.  Cheers!