String Comparision Weirdness

We had major problems after migrating the DB to a more powerful server; we
managed to locate the problem to a type conversion bug in our software.
Never the less, this thing puzzles us a lot:

NBTEST2=# select '-1'>'0';
?column?
----------
t
(1 row)

We've tried this query on several servers with different versions of
postgresql and different versions of glibc - some returns true, others
returns false - and it seems neither to be related to the postgresql version
nor the glibc version. At all servers we tested, strcmp("-1","0") returned
negative - at some -3 and at others -1, and not related to postgresql.

The correct result above should be false, since ascii('-')=45 while
ascii('0')=48.

Can the character set in use be significant?

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