So apparently, "strait" is an old word meaning "tight." The slang, "That's strait" evolved from "That's tight." Therefore, "that's strait" is slang that evolved from other slang through the substitution of an archaic meaning for a synonym. What crazy English-major thug thought that one up? Still, imagine all the fun to be had with saying girls are "straight"/"strait." There's no difference in pronunciation and rather different meanings.
Also, I've learned that "disinterested" means neutral. "Uninterested" indicates a lack of interest. "Disinterested" used to mean "uninterested" when it was first coined, but the meaning has shifted and now the old is considered incorrect. I always thought they were the same.
English is awesome sometimes.
On a related note,
http://www.wsu.edu/~brians/errors/errors.html is a really good link.