Did this message show when attempting to run the installer , or did that go w/o incident, and the message comes when opening the new Safari ?.
If you do a get-info in Finder on Applications/Safari, it shows 5.0.6 ?
About this Mac shows 10.5.8 ?
If it won't install & the OS version is correct, there's likely a mismatched
/System/Library/Frameworks/WebKit.framework from attempting to use an incorrect Safari version... using the shareware Pacifist to 'force' the install of the 5.0.6 package usually takes care of that w/o needing to reinstall OS X or manually deleting the framework. As always, back up first.
I got the message when attempting to run the installer. My Mac is 10.5.8., but I read somewhere that I should be using Safari 5.0.6 (although there was no trace of Safari in Finder).
Having said all that, I followed your advice re: Pacifist, without having a clue about what I was doing (not exactly a techie!) or without backing up, and got Safari back. It kept crashed but once I rebooted it seems to be fine -- so, thank you very much!