In a Google Webmaster Help thread, Googler, JohnMu, might be stating the obvious, but it is always good hearing it from someone who is in the know.
JohnMu, answered a question about links, but in his answer, he confirmed that even with or without sever header responses (301s, 404s, etc.) Google will typically treat the home page of a site, i.e. /, the same if it were / or /index.html and likely index.php or any other extension.
Given that generally URLs like "/" and "/index.html" are the same, we tend to treat them the same (unless there's a good reason not to do so -- say when there is unique content on them).
Clearly, if the pages look different and have different content, Google will notice and likely treat them differently. It is still good practice to 301 redirect the canonical URLs to a single URL, but most webmasters have no clue about these issues and Google has to be smart enough to figure these issues out themselves. In most cases, they are.
Forum discussion at Google Webmaster Help.