In all other major browsers(all Mozilla-based browsers, Opera, and Explorer) where tabs are set to be used, if window.open is called without any size parameters, the new page opens as a new tab in the current window. In the version of Safari 3.1 that is current as of 2008-06-03, a new page will open in a new window instead.
Tabs are a Safari (the browser) feature, not WebKit. It's up to Safari or any other application that embeds WebKit to decide how it handles new windows and tabs and such. The appropriate place to request this enhancement is at <https://bugreport.apple.com/>.