g1smd - 9:29 pm on Jan 20, 2013 (gmt 0)

REQUEST_URI is updated to point to a different internal resource after an internal rewrite.

THE_REQUEST contains the original
GET /thisthing HTTP/1.1
HTTP request and is not changed as rewrites are processed.

You need to look at THE_REQUEST to be sure that you're looking at what was requested by the browser, rather than as a result of a previous internal rewrite.

1. Request example.com/index.php non-canonical URL.
2. Redirect to www.example.com/ canonical URL.
3. Internally rewrite to /index.php internal path to fetch the content.

If you test REQUEST_URI instead of THE_REQUEST in (2) then when step (3) is executed, the reparse of the htaccess file will re-match the redirecting rule (htaccess is reparsed until no more rules match the current request), invoke it and expose the previously rewritten internal path back out on to the web as a new URL. Requesting example.com/index.php will redirect to www.example.com/ and then requesting www.example.com/ will internally rewrite to index.php and then immediately redirect to www.example.com/index.php. The browser requests www.example.com/index.php and around you go again. You've now got an infinite loop.

Apologies for the typo yesterday. The [^ ]* should have been [^\ ]* as Lucy pointed it.
It was well after midnight here.