Here's how to hack Safari's "Add Bookmark" toolbar button into a "New Tab" button. This old hint from activewin.com doesn't work any more due to the new architecture.
Disclaimer:b the method here worked for me and I make no guarantees that it'll work for you, even if it should. Don't blame me if your shiny new OS X goes to bits because of this!)...
- Backup Safari.
- Install XCode (from the OS X DVD). A bit of overkill for that one button, yes.
- Open Safari's package contents (control-click and select Show Package Contents)
- Navigate into Contents/Resources/English.lproj
- Open ToolbarItems.nib (double clicking should open with Interface Builder)
- In the 'ToolbarItems.nib (English)' window, double click on 'First Responder.'
- Press Command-Shift-I to bring up the Inspector window, if it's not already open
- In the Inspector window, press Command-1 to make sure you're looking at 'Attributes.' Click on Add, and create a new item called newTab.
- In the 'Toolbar Items' window, click on one of the 'torn pictures' and press Command-Shift-I to bring up the Inspector window if it's not already open.
- Select the one nearest the Address bar (this should correspond to the placement in the Customise Toolbar menu); this is the 'Add Bookmark' button.
- Press Command-2 or choose Connections. Select 'newTab' and click 'Change Action' or simply double-click on it; there should be a round pushbutton icon next to it now.
- Save, quit and test.
[robg adds: I haven't tried this; I find the Command-T shortcut is quick enough for me...]

