Hm, you're right -- could be that cpu/swappiness was the issue. If instantiate less from the lazy sequence, I can indeed break with ctrl-c, so eg: 30000 is about enough not to eat all resources, and still take enough time that I can ctrl-c--out (and also does complete without crashing if I let it run a little while, about a second I think):

    (take 1 (reverse (take 30000 nats)))

Anyone know if this is a known issue with Haskell (I assuming this is a pretty straight wrap for Haskell lazy lists?) -- that you can get the compiler/runtime into a tight loop eating so many resources that it isn't possible to abort cleanly?