If here's the relevant code from the default search template:
http://www.movabletype.org/default_templat...ch_results.mtml<MTSearchResults>
<MTBlogResultHeader>
<h1 class="search-results-header">
<MTIfStraightSearch>
Results matching “<$MTSearchString$>” from <$MTBlogName encode_html="1"$>
</MTIfStraightSearch>
<MTIfTagSearch>
Results tagged “<$MTSearchString$>” from <$MTBlogName encode_html="1"$>
</MTIfTagSearch>
</h1>
<div class="search-results-container">
</MTBlogResultHeader>
<$MTInclude module="Entry Summary"$>
<MTBlogResultFooter>
</div>
</MTBlogResultFooter>
</MTSearchResults>
I take it this is looking for the module "Entry Summary" to write the search results code? But, if you're upgrading from MT 3 as I did, there is no such module, so it breaks the search results, unless I'm missing something.