I'm going to assume that you're doing this in a cell-based NSTableView, because NSButtonCell does expose a title binding, but it's useless in the context of an NSTableView, because you have to bind the NSTableColumn, and that only exposes a Value binding for NSTableColumns containing NSButtonCells. It's a shortcoming of the framework, and one that Google reveals that many have grappled with.
One approach to this is to update the title from the model in your -tableView:dataCellForTableColumn:row: and -tableView:willDisplayCell:forTableColumn:row: delegate methods. This has the drawback of not being updated via KVO when the underlying model changes. If that's important for your application, one workaround is to make the model keyPath for the value of the checkbox dependent on the model keyPath for the title of the checkbox. This way, when the title changes, KVO will send notifications for a change to the value, which will cause the row to be re-drawn, and thus the title will be updated by the delegate method.
I notice that this question is old, but an easier approach these days would be to use NSView-based NSTableViews. When done that way, you're working with a first-class NSButton control, with both Value and Title bindings (and a bunch of others) available. It's often a more workable approach, assuming you're not worried about supporting OS version prior to Lion.
