[ruby-core:35777] hashes are not consistent across ruby processes?

Hello all.
I have a small program here that caches lookups.  To do so it writes
out a "unique" file based on the current ruby script, ex:

filename = File.expand_path("~/#{$0.hash}")

This worked great for looking up the file in 1.8.x, however in 1.9.x I
see the following behavior:

c:\dev>irb
>> 'abc'.hash
=> -753597542
>> 'abc'.hash
=> -753597542 # so it is consistent within the same process

now run irb again:

c:\dev>irb
>> 'abc'.hash
=> 552773397 # different now!

The hash values change each time you run a new process.  I would have
expected them to stay consistent so this is a bit surprising (even the
hash values for things like an empty array are different now, too, and
didn't used to be).

Any feedback on this?
Thanks.
-r