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I think that exposing pdb's namespaces for debugged code is
dangerous.When debugged code have this kind of access he can
dynamic change pdb's behaviour without your control:

y.py:
die = """\
def destroy(x,y):
        print 'Iam crashing your HOME and deleting your FILES'

Pdb.__dict__['do_break'] = destroy # pdb's break = destroy
"""
x.py:
# innocently looking code;)
import y
exec(y.puff)
print "X"

with your patch:
$ python2.5 -m pdb x.py
> /home/xyz/python/x.py(1)<module>()
-> import y
(Pdb) Pdb.__dict__['do_break']
<function do_break at 0xb7cafdf4>
(Pdb) break
(Pdb) n
> /home/xyz/python/x.py(2)<module>()
-> exec(y.puff)
(Pdb) n
> /home/xyz/python/x.py(3)<module>()
-> print "X"
(Pdb) Pdb.__dict__['do_break']
<function destroy at 0xb7cb81b4>
(Pdb) break
Iam crashing your HOME and deleting your FILES

I think that this patch can't be accepted due to above
reason.According to my advanced reaserch;) ( find Lib/ -name
'*.py' -exec grep 'from __main__ import' {} -ls \; ) 'from
__main__' is rare case so maybe it will be reasonable to
simply handle ImportError and print something like
'** 'from __main__ import' not supported' message.What do  
you think?