In Safari 3, you can now Command-click on a bookmark inside a folder on the bookmarks bar to open it in a new tab. In the old version, you could only do this for bookmarks at the top level of the bookmarks bar, not inside folders. If you Command-click on the folder at the top level in the bookmarks bar, it performs an Open in Tabs operation.

After checking further, it now works for any bookmarks, even in the main bookmarks menu. I'm not sure if you get this behaviour in the Safari 3 beta or just in Leopard.

[robg adds: I tested this in 10.4, and I believe it works there as well as in 10.5.]