I thought about this, but couldn't see the obvious way of determining the "main" file.

For example if you run:

wget --page-requisites http://en.wikipedia.org/

The output produced is:

en.wikipedia.org/
|-- robots.txt
`-- wiki
    `-- Main_Page

Determining that wiki/Main_Page should be transformed to index.html is hard..

Just do:

    wget --server-response  http://en.wikipedia.org/
[download]

and you can parse out the redirects:

--13:13:55--  http://en.wikipedia.org/
           => `index.html'
Resolving en.wikipedia.org... 66.230.200.100
Connecting to en.wikipedia.org|66.230.200.100|:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.0 301 Moved Permanently
  Date: Thu, 25 Jan 2007 18:13:41 GMT
  Server: Apache
  X-Powered-By: PHP/5.1.4
  Vary: Accept-Encoding,Cookie
  Cache-Control: s-maxage=1200, must-revalidate, max-age=0
  Last-Modified: Thu, 25 Jan 2007 18:13:41 GMT
  Location: http://en.wikipedia.org/wiki/Main_Page
  Content-Type: text/html
  X-Cache: HIT from sq28.wikimedia.org
  X-Cache-Lookup: HIT from sq28.wikimedia.org:80
  Age: 14
  X-Cache: HIT from sq26.wikimedia.org
  X-Cache-Lookup: HIT from sq26.wikimedia.org:80
  Via: 1.0 sq28.wikimedia.org:80 (squid/2.6.STABLE9), 1.0 sq26.wikimed
+ia.org:80 (squid/2.6.STABLE9)
  Connection: close

  ---> Location: http://en.wikipedia.org/wiki/Main_Page [following]

--13:13:55--  http://en.wikipedia.org/wiki/Main_Page
           => `Main_Page'
Connecting to en.wikipedia.org|66.230.200.100|:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.0 200 OK
  Date: Thu, 25 Jan 2007 18:13:44 GMT
  Server: Apache
  X-Powered-By: PHP/5.1.4
  Content-Language: en
  Vary: Accept-Encoding,Cookie
  Cache-Control: private, s-maxage=0, max-age=0, must-revalidate
  Last-Modified: Thu, 25 Jan 2007 17:28:15 GMT
  Content-Type: text/html; charset=utf-8
  Age: 11
  X-Cache: HIT from sq30.wikimedia.org
  X-Cache-Lookup: HIT from sq30.wikimedia.org:80
  Via: 1.0 sq30.wikimedia.org:80 (squid/2.6.STABLE9)
  Connection: close
Length: unspecified [text/html]
[download]