I am dealing with mod_rewrite for Home windows (with an ISAPI filter) and have not yet seen documentation regarding how to setup RewriteRule allowing the present url without any changes.

RewriteCond %{HTTP_USER_AGENT} !.*iPhone.* [NC]
RewriteRule () http://DontWantToRedirect/orAlterURL [R,NC,L]

Let me produce a rule allowing only apple iphones with OS 4..1 for connecting to my server. You will find other rules in play here, so I must keep your logic if - then - passthrough w/no change

Not to rewrite a URL with RewriteRule try simply using a dash because the rewritten path...

RewriteRule ^.*$ - [NC,L]