The bug can be reproduced by starting the textpad
module directly (by executing 'python textpad.py'
inside the curses directory). 
The you should type:

1
2
3
C-a
C-k

You would expected an empty textbox (because you first
enter the string '123', the go the the beginning of the
line and then delete until the end of the line). But
the textbox contains the text '12'.

My Python version is 2.3.3, a patch is attached.