That has been removed in Safari 3 for some reason, but the basic functionality is still there.
Thanks to osxhints I can now share with you how to make that work…
1. Open the Safari plist file (~/Library Â» Preferences Â» com.apple.Safari.plist) in the Property List Editor.
2. Click the triangle to open the Root list, then click once on Root to highlight it.
3. Click the Add Child button.
4. Change the name (New item) to AlwaysShowTabBar, set the Class to Boolean, and set Value to Yes.
5. Save changes and quit Property List Editor.
Note: Changing Yes to No will stop always showing the Tab Bar for those of you who upgraded from Safari 2.
It’s a bit of a hack to be sure, and I hope that in a later Beta of Safari 3 that they put that option back in, but for those of you (like me) that just can’t wait, this is a solution that will get it fixed for you.