When it isn't released yet? The general opinion is that Apple will open up the iPhone to third-party developers in October by providing a software development kit (SDK) with Leopard. I don't know anyone that isn't expecting this to happen now that the API has been more or less reverse-engineering and the first "professional" third-party applications are seeing the light of day.
Nobody is selling anything yet, but its interesting to see companies like Delicious Monster are willing to release native applications. Sociologically at least, this is a whole different kettle of fish than individuals releasing terminal emulators and other such stuff. Companies, even uber-cool ones are notoriously conservative. They generally take legal advise before doing stuff like this, and they're presumably pretty sure that Apple legal won't be dropping by to say hello in the near future. Which is interesting, no?