Came across troubleshooting an issue with a site collection with the name bin. This site collection had a url like http://portal/sites/bin, making the default homepage http://portal/sites/bin/default.aspx. For some reason the site did not render and I got a HTTP 404 error. After analysis of the ULS logs not showing any trace of the request and the analysis of the IIS logs, I wnet looking for an answer on the interwebz. Stumbled upon the following article from Russ Michaels : http://www.michaels.me.uk/post.cfm/iis7-blocks-viewing-access-to-certain-folder-names
As it appears IIS 7 blocks access to urls where /bin/ is present in the Url.
The solution to this problem is to rename the site collection or remove the exception in the web.config of your web application as suggested by Russ.
I prefer the web.config approach on the web application level by adding the following section:
<?xml version="1.0"?>
<configuration>
<system.webServer>
<security>
<requestFiltering>
<hiddenSegments>
<remove segment="bin" />
</hiddenSegments>
</requestFiltering>
</security>
</system.webServer>
</configuration>