I think the Nike Air Streak Ekiden and/or Air Streak Spectrum Plus (virtually identical to each other) are probably the most similar to the Rats Racer. You'll probably notice that the Nikes are a little firmer feeling, and have a bigger, stiffer plastic plate in the arch. Other than that, they're quite similar.
The Cubato is quite a bit lighter and "floppier" (a nice shoe, but not really anything like the Rats Racer).
The Streak might be sort of similar except for the medial posting, but I think the most similar Nike would probably be the Ekiden or Spectrum.
I don't know anything about the NB 750. The NB 550 and Asics Tiger Paw might also be worth a try. They're comparable to the Rats Racer, but I think both are a little more flexible and less bouncy feeling.
Some of the newer Adidas racing flats (Response Comp?) might be worth checking out. I don't know anything about any of them, except that the medial post on the Supernova Comp is ridiculously big and firm. I like the midsole feel of most Adidas shoes, and I think that'll be hard to find in any of the Nike, NB, or Asics flats that I mentioned.