I had an issue with crawling a site recently - the crawl would stop immediately it started with the message :-

The start address http://servername/default.aspx cannot be crawled.
Context: Application 'Search index file on the search server', Catalog 'Search'
Details
Access is denied. Check that the Default Content Access Account has access to this
content, or add a crawl rule to crawl this content. (0x80041205)
No results matching your search were found.

Joel Oleson ( http://blogs.msdn.com/joelo ) found a KB which described a similar problem :-

KB896861 at http://support.microsoft.com/kb/896861

The issue was Loopback Checking, which IIS uses to protect against Denial of Service Attacks. Basically, the default value does not allow the Search Service to crawl the site, because it essentially receives a dangerous level of Requests coming from the loopback address (default 127.0.1.1), which it denies access to "thinking" it is an attack.

I am finding this problem becoming more common and more prevalent since the Infrastructure Update and later patches were installed.  This may just be co-incidence.

See Joel's blog for more solutions and workarounds.

http://blogs.msdn.com/joelo/archive/2007/06/14/i-m-indexing-and-i-only-get-errors.aspx