Just van Rossum suggested I file this bug report:

When building with
    python -O buildapp.py -v --standalone --
package=encodings --package=_strpTime --packag
e=urllib2 build
I got these errors:
    'import site' failed; use -v for traceback
    Traceback(most recent call last):
    
file "./build/MyApp.app/Contents/Resources/myapp.py", 
line 6, in ?
        import copy
    ImportError: No module named copy

The "import copy" problem is simply cascaded off 
the "import site" problem. A site.pyo file was generated 
in the /build/MyApp.app/Contents/Resources/folder, but 
it is obviously not being used properly.

To get it to work, I first built the application without 
the -O flag, saved the site.pyc file, then built it again 
with the -O flag and restored the site.pyc file.  With this 
hack, the application works fine.

Just responded:
I think you need to hack the bootstrap script by adding
    sys.argv.insert(1, "-O")
after this line:
    sys.argv.insert(1, mainprogram)

He also asked if -O was really worth the trouble, but 
that should be the programmer's decision, not the 
framework's.  bundlebuilder should support --standalone 
with -O.

Fixed in r68163