Possible Duplicate:
Open URL in new Safari tab with AppleScript
How do I tell Safari to open a URL in a new tab of the current window?
This is a duplicate of Open URL in new Safari tab with AppleScript, however its answer no longer works with the current version of Safari (5.1 on 10.6.8). The tab is created but empty:
tell front window of application "Safari"
make new tab with properties {URL:"http://www.stackoverflow.com"}
end tell
If you make a new "document" instead the URL is honored, but this opens a window.
For what it's worth, I reported this as a bug to Apple. But a workaround would be great.