Event Source: Windows SharePoint Services 3 Search
Event Category: Gatherer
Event ID: 2436
The start address
Context: Application 'Search index file on the search server', Catalog 'Search'
Access is denied. Verify that either the Default Content Access Account has access to this repository, or add a crawl rule to crawl this repository. If the repository being crawled is a SharePoint repository, verify that the account you are using has "Full Read" permissions on the SharePoint Web Application being crawled. (0x80041205)
This stumped me for a long while until I finally took the time to dig in and do the proper research. I found another error showing in the SharePoint log file:
CSTS3Accessor::GetDocFormat: Return error to caller, hr=8007007A - File:d:\office\source\search\search\gather\protocols\sts3\sts3acc.cxx Line:654
A simple Google search turned up this post by Yorick with the solution.