According to discussion with you.you need to do this by using layout xml
Home handler: cms_index_index is unique for homepage.
Suppose that the name of custom header is customheader
then you can define the most_viewed by
<cms_index_index>
<reference name="customheader">
<block type="mostviewed/list" name="home.mostviewed.list" as="product_homepage" template="catalog/product/mostviewed.phtml" />
</reference>
</cms_index_index>
Then you need call this block by using getChildHtml(BlockNameATXmlCOde)
<?php echo $this->getChildHtml('product_homepage');?>
Please put this code at cms.xml or local.xml at app/design/frontend/YOUR_PAcakage/YOUR_TEMPLATE/layout/
If you using layout code then no need to cms block
Hi you need to change reference name custom header
<reference name="header"> <block type="page/header" name="customerheader"> <action method="setTemplate"><template>cms/home.phtml</template></action> </block></reference>
Then accordingly you need to change most view block reference block name
<cms_index_index>
<reference name="customerheader"> <!-- here parentblock name which is define at above xml -->
<block type="mostviewed/list" name="home.mostviewed.list" as="product_homepage" template="catalog/product/mostviewed.phtml" />
</reference>
</cms_index_index>