Though the spell checker does not learn regularly, there are ways to get irregular words to be read as 'correctly spelled" by the iPhone.
The spell checker feature inherent in the iPhone is both a blessing and a curse for those sitting down to perform the arduous task of typing on their keyboard. Often times the suggestions that they give are so far outside the bounds of normal thought that more often than not people will ingore them by pressing the X in the corner instead of accepting the change. Even if you ignore or do not notice the spell warning, it will forcefully change the word for you. You can always turn the spell check off, but instead it is better to teach your spell checker the words it does not know already.
Teaching the Spell Check Feature
For many situations, the spell checker will actually learn certain words just through your actions. After rejecting the suggestions for the same word many different times it will begin to realize that the word is intended to be spelled in that fashion. This usually only works temporarily and will not work at all for some words. This is adequate for some situations, but for many you are going to have to take the contribution a step further.
Trick for Using Your Contact List
Certain words, such as some slang and swear words, will never be recognized as regular words. There is a way to get around this entirely, and to correct any other word issues that the iPhone is having. One of the word resources that the iPhone references with its typing spell check is the contact list. If you use certain words often that the spell checker has problems with you can create a contact for that word in your contact list. As long as the word is somewhere in your contacts list it should still work.
Good... For Now...
This may not fix all the problems that have come with misspelling and spell check inaccuracies, but it can on proper names, swear words, and over-used casual lingo. So far, there is not a proper learning or contributing function for the iPhone's spell checker, but there is bound to be in future updates. Until then you must take an active role to make the awkward typing functions work for instead of against you.