Run AppleScript When Webpage is Done Loading

This AppleScript code will check if the Safari's frontmost window is done loading:

tell application "Safari"
delay 0.5
repeat until ((do JavaScript "document.readyState" in document 1) is "complete")
delay 0.25
end repeat

-- Action to perform once page is loaded.

end tell

This works with Safari 4 in both Leopard and Snow Leopard.