I have something exactly the same but I build an XML editor - the widgets are determined by the data types from the schema (spinboxs for integers, text boxes for strings etc) and my code builds these widgets as required by the XML file You'll find yourself using closures a lot to handle the callbacks - most of my methods return subroutines that get assigned to callbacks (-command or -validatecommand params for the Tk widgets) - the closure part comes from those subs having references to data from the time of their creation, to be used at the time their actually called - sounds like a time-warp waiting to happen, but its actually pretty cool.
And dont forget that things like Firefox are built out of their XUL description - the firefox code interprets the XUL and builds the browser...so its a pretty common, albeit advanced, technique
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