
I have a UIBarButtonItem in my UIToolbar titled
Assuming you want to support iOS4 and earlier, your best bet is to create a bar button using the initWithCustomView: method and supply your own view which could be something like a UIButton where you can easily customise the font.
You can also drag a UIButton onto a toolbar or navigation bar in Interface Builder if you want to create the button with drag-and-drop instead of programmatically.
Unfortunately this means creating the button background image yourself. There's no way to customise the font of a standard UIBarButtonItem prior to iOS5.