AFAIK you can't do this easily in .htaccess. I spent a while banging my head on it. There are solutions if you want to hardcode your base url but this does not work for the framework so not suitable.
See http://open.silverstripe.org/ticket/6700 for a bit of a discussion on this as well as http://pastie.org/2373435. It doesn't trim the extension off like it probably should (didn't notice that behaviour).
Hmm, maybe I spoke too soon. When I try to login to the admin by appending /admin I get this error:
Fatal error: Call to a member function getExtension() on a non-object in /hsphere/local/home/client/mywebsite.co.uk/mysite/code/Page.php on line 36
Anyone got any ideas how I could write an exception to allow the /admin sudo-folder to work? Otherwise it's working like a charm.
Well my SEO pro is also not so happy about alot of stuff SS is doing, stuff like http://www.nobrainer.dk/?url=services and http://www.nobrainer.dk/index.php/services gives same page as http://www.nobrainer.dk/services and http://www.nobrainer.dk/services/ the missing trailing backslash i have fixed in the htaccess, but this stuff should really be fixed in core by advice from a SEO pro.