From the docs:
all system-generated hidden fields are
rendered at the top of the page. This
makes sure that the information in
these fields is sent to the server
even if a postback is performed before
the page has finished loading. If
RenderAllHiddenFieldsAtTopOfForm is
set to false, performing a postback
before the page has finished loading
can cause an "Invalid postback or
callback argument" error.
Although the property name indicates
"all hidden fields," user-created
hidden fields are not affected by
settings for this property.
I'm not sure what you are attempting to accomplish here, but it doesn't look like you can do what you are wanting to. And, for that matter, I'm not sure it's a good idea even if you could.