So I moved Safari 5.1 to my Snow Leopard disk and was told it would only run on 10.7. So I went into it's info.plist and changed it to 10.6.7. It still says it won't launch.
You could try running the current version of Safari for SL. The version of Safari that came with Lion isn't meant to be run on SL and likely will take a lot of hacking to get it to work.