I really like this option. Anecdotally, it looks like *most* raw string users are really writing regex's. Then, this option will do the right thing for 90%-95% of users, and the other 5%-10% of users will have a very simple way workaround.
As mentioned, none of us have actually bumped into the "R" prefix in use, so this won't effect existing code containing regular expressions. It will effect existing code containing raw strings that aren't regular expressions, but these are in the minority.
OK, This has been sitting for a couple of days...
So, I'm in favor of using the capital "R" prefix to 'turn off' regex highlighting in raw strings. Raw strings with the lower case "r" prefix will be considered to contain regular expressions as before. I'm going to go ahead and implement it tomorrow, unless there are some strong objections tonight.
–Alex