2 Suggestions:

1. It'd be very nice if you had a version that tweaks text colors and doesn't touch anything else, i.e. just like on the demo page. When I tested your bookmarklet and it tore up the page I thought something was broken. I only found out that it uses readability because I started digging when it 'broke', you never mention 'readability' in your copy.

2. People will want it enabled by default. You can't do that if you use readability.