If you use the Three20 library, I think the TTButton toolbarButton style is what you want. TTButton is a subclass of UIButton, but it allows you to apply styles the same way you use css when creating a web page.
To create a button like UIBarButtonItem, just call
[TTButton buttonWithStyle:@"toolbarButton:" title:@"Title"]
You may need to save it because the button is the subject of an automatic release.
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