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I think 2's complement makes good sense for arbitrary 
precision longs.  This is how OpenSSL and GMP handle them.  
It's also how the ASN.1 BER/DER encodings handle integers: 
these encodings just prepend tag and length fields to the big-
endian 2's complement value.  I.e.: If you want to extract 
RSA public values from an X.509 certificate, they'll be in 2's 
complement (well, they'll always be positive... but they'll 
have an extra zero byte if necessary).

Since the functionality for 2's complement is already in the C 
code it's easy to expose through a patch.  So I'm still in favor 
of presenting it.