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I'm on Windows with cp1252 as the default encoding.
When I use -*- coding: c1252 -*- I get no problems.
When I use -*- coding: utf-8 -*- IDLE -r still opens the file with cp1252 encoding.
Python.exe opens the file with utf-8 correctly.
I think the problem is in Python32\Lib\idlelib\PyShell.py line 585:
In class ModifiedInterpreter:
 def execfile(self, filename, source=None):
 "Execute an existing file"
 if source is None:
 source = open(filename, "r").read() # this is the bug IMHO

The problem can be fixed with tokenize :
I'm sorry I never submitted a path and I have no access to the source tree from here, if someone cares to do it, do not hesitate.
 def execfile(self, filename, source=None):
 "Execute an existing file"
 if source is None:
 import tokenize
 source = tokenize.open(filename).read()

Yes, tokenize.open() should fix this issue, but you should close the file after using it. Use for example "with tokenize.open(): ...".
Can you write a patch? You can download the source code using Mercurial or download it manually from http://hg.python.org/cpython/file/tip
See also our http://docs.python.org/devguide/ 

Here is the patch:
diff -r e8da570d29a8 Lib/idlelib/PyShell.py
--- a/Lib/idlelib/PyShell.py Wed Jul 27 21:28:23 2011 +0200
+++ b/Lib/idlelib/PyShell.py Wed Aug 31 20:16:38 2011 +0200
@@ -582,7 +582,9 @@
 def execfile(self, filename, source=None):
 "Execute an existing file"
 if source is None:
- source = open(filename, "r").read()
+ import tokenize
+ with tokenize.open(filename) as filein:
+ source = filein.read()
 try:
 code = compile(source, filename, "exec")
 except (OverflowError, SyntaxError):
Sorry for taking such a long time, I was on holidays.

New changeset fe7777f1ed14 by Victor Stinner in branch '3.2':
Issue #12636: IDLE reads the coding cookie when executing a Python script.
http://hg.python.org/cpython/rev/fe7777f1ed14
New changeset a8748022504f by Victor Stinner in branch 'default':
Merge 3.2: Issue #12636: IDLE reads the coding cookie when executing a Python script.
http://hg.python.org/cpython/rev/a8748022504f 

Fixed, thanks for the report.
Python 2.7 is not affected by this bug.