so what you are saying is when you go to:
http://example.com/index.html
your Rewrite rule is applied. But when you go to:
http://example.com/
your Rewrite rule is not applied. If thats the case then that is the default behaviour and how it works with my apache 1.3 server. With mod_rewrite you have to specify the exact pattern to match, if the text in the request string does not exactly match the pattern the the rule will not be applied.
I suggest adding a RewriteRule for the case where the user only requests the root with a url like this:
http://example .com/
Try this to match the root of your web server
RewriteRule ^/$ /index.html
if you want this rule to rewrite all index files no matter what directory they are in try this
RewriteRule ^/(.*)/$ /$1/index.html
Hope that helps you out
Aaron