__import__() raises an ImportError "No module named ..." if the module 
file is not readable or module directory is not executable. The 
expected message is something like "Module named ... is not readable".

Directory tree for the test:
------------------------------
marge$ ls -lRF .
.:
drwxrwxrwx 2 haypo haypo 4096 2009-02-26 16:51 module/
-rw-rw-rw- 1 haypo haypo    9 2009-02-26 16:48 module2.py
-rw-r--r-- 1 haypo haypo  414 2009-02-26 16:50 test.py

./module:
-rw-r--r-- 1 haypo haypo 10 2009-02-26 16:45 __init__.py
------------------------------

Success with correct file permissions:
------------------------------
$ python test.py
__import__('/home/haypo/xxx/module') ...
PI = 3.14
__import__('/home/haypo/xxx/module2') ...
E = 2.72
------------------------------

Change module/ and module2.py permissions to 000 (no read, no write, 
no execute):
------------------------------
__import__('/home/haypo/xxx/module') ...
No module named /home/haypo/xxx/module
__import__('/home/haypo/xxx/module2') ...
No module named /home/haypo/xxx/module2
------------------------------