@SJK: that info is misleading. The plist setting may be experimental but has worked from Safari 3 up (now at Safari 4.0.4). And for me, it also catches scripted windows or popups. So well, in fact, that I am once in while tempted to reverse the setting because legitimate popups also open in new tabs.
BBC Radio Widget
+4
SafariSingleWindow
+1
+8
diskring