File a feature request with Apple and let them know you want this functionality on the iPhone. NSDateFormatter provides a legacy init method that takes a boolean flag indicating you want it to parse natural language, but it's only available on OS X. Wil Shipley wrote an interesting post a while back about this functionality in the context of heuristics and human factors.
It doesn't seem likely that Apple will provide this functionality as this note would indicate from the NSDateFormatter docs:
iPhone OS Note: iPhone OS supports
only the 10.4+ behavior. 10.0-style
methods and format strings are not
available on iPhone OS.
In other words, I think you'll have to parse it yourself.