Back in December 2008, I came up with an "interesting" solution to iOS's annoying habit of "correcting" the word that I wanted with the word that it thought I wanted. The problem was colloquially known as the "Ducking iPhone" for reasons you are probably smart enough to figure out on your own. (If you're confused, here's the PG-13 version, or, for the more sensitive types, TUAW also has a PG-rated version.)
The solution was simple: Create a new contact in your iOS address book, using the word you want iOS to learn as the first or last name. The good news is iOS is much better about learning words (tap the 'x' which appears when the auto-correct option appears),