Put the regular "Rounded Rect Button" (UIButton) on toolbar (not "Bar Button Item")! Be aware, that behind the scene, IB wrap UIButton with UIBarButtonItem. It is why you have to click on button twice to get to UIButton properties (true for Xcode 4.2).
Get to UIButton properties. When you click first time, you will get "Bar button item" properties (it is what IB created automatically for you). Now click second time, you'll get to "Button" properties (also, when you selected UIButton you can't resize it and can't see resize markers).
In the Button properties set type to "Custom" to remove rounded rect border around button (you may need to resize button in IB to refresh it before you can see difference).
In the Button properties under "View" uncheck "User Interaction Enabled".