On MacOS X 10.3.7 using the built-in python 2.3 and Aqua Tk 8.4.9, 
tkSimpleDialog.askfloat() with initialvalue specified misbehaves:
- The entry box is displayed over the prompt.
- Clicking on the entry box has no effect.
- Clicking below the entry box (i.e. where it OUGHT to be displayed)
  acts as though you clicked in the entry box.

If you omit the initial value then the bug does not appear.

Also: on one particular unix system, similar code caused the entire 
Tkinter application to become unresponsive once the dialog box was 
dismissed. However, the parent window for that dialog was not root 
and I did not specify the parent argument to askfloat, so that may 
explain the hang.

See the attached sample code for details.