I've released two packages to help make Tk applications fit in more "natively" on Mac OS X.
The Mactoolbar Package
The mactoolbar package is designed to emulate the standard Cocoa toolbar provided on OS X. On the Mac, toolbars behave a bit differently than they do on Windows or X11; rather than having the entire button darken when pressed, only the image and/or text darkens, and the toolbar itself can be hidden or made visible with a button-click. As such, the usual Tk approach to toolbars (arranging Tk or Tile buttons in a frame) requires a bit of customization. The mactoolbar package uses a Tile frame, labels, and bindings to implement a Cocoa-style toolbar.
The aquahig package implements some Mac OS X-specific configurations to selected Tk megawidgets to make them better fit in visually with Mac interface conventions, similar to iTunes and other Mac applications. Currently supported widgets are the BWidgets Tree, the BWidgets ListBox, and Tablelist.