The replacement file, for anyone without a dev setup, is
http://hg.python.org/cpython/file/cc86f4ca5020/Lib/idlelib/PyShell.py

After renaming PyShell to PyShellBak and replacing with the above,
IDLE seems to run better than ever. On my XP system, the several second delay for the old process dying is gone and I never see 3 pythonw processes, even temporarily, as before. I can no longer tell from TaskManager that one process has been killed and another started. Testing included running tk windows. Will test later on new Win7 system.

Pending some unforeseen new problem, thank you Ned and Eli.