jdMorgan - 4:53 pm on Sep 18, 2010 (gmt 0)
That code is an internal URL-path-to-filepath rewrite, not an external client URL-to-URL redirect. Don't confuse the two, as it makes understanding and using mod_rewrite much more difficult.
As well as mod_speling, you could be getting interference from mod_negotiation. If not required by your site, disable content negotiation by adding "-MultiViews" to the Options directive likely already at the top of your .htaccess file. For example:
Options +FollowSymLinks -Indexes -MultiViews
is a "typical" setting when using mod_rewrite.