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Well, I'm stumped.  I checked the diff from when 2.2 was initially 
released until now and the only change that seems to be related 
to any of this is that what is returned by currentThread is not 
saved in a variable.  But since the error is the calling of 
currentThread itself and not saving the return value I don't see 
how that would affect anything.

I also went through offlineimap but I didn't see anything there 
that seemed to be setting currentThread to None.  Although 
since several files do ``import *`` so there still is a possibility of 
overwriting currentThread locally.

So, for my owning learning and to help solve this, I have written 
a tracing function that writes to stderr using the logging package 
when it detects that either currentThread or 
threading.currentThread has been set to None, locally or globally 
(I assume the code is not injecting into builtins so I didn't bother 
checking there).  The file is named tracer.py and I have attached 
it to this bug report.  If you can execute 
``sys.settrace(tracer.trace_currentThread)`` before offlinemap 
starts executing and immediately within each thread (it has to be 
called in *every* thread since tracing functions are no inherited 
from the main thread) it should print out a message when 
currentThread becomes None.  If you *really* want to make this 
robust you can also have it check sys.modules['threading'] every 
time as well, but I figure there is not going to be much renaming 
and masking of currentThread.