Testing the latest dev version of the WP Super Cache plugin and using the mobile rules, I noticed that the htacess rules are using two different cases for the user_agent.
%{HTTP_USER_AGENT} and %{HTTP_user_agent}
If you copy the original two lines after you activate the mobile support, both lines use the lower case _user_agent. The plugin then shows the full rewrite rules with the first line needing to be the all upper case version and the second line the lower case. In TextMate, it highlights the lower case line as if it's improper.
I don't know the first thing about htaccess rules and case sensitivity, so I figured I just point out my observations.
