Forget the iPhone. Forget Android, which may not quite deliver. While these smart phones and mini-computer devices are becoming a reality, why not go old school? Make your application SMS accessible. Text messaging is available on darn near every phone available today, even the boring candy bar ones that providers give away for free.
There are several services with shared short codes, those five digit numbers like Google's 46645. Getting your own short code costs thousands of dollars and comes with the political mission of getting accepted by each carrier separately. With a shared code, someone else has already done the hard work. You reserve a keyword on their service, so that when you text 'webmonkey' to 66937, for example, users receive a (hopefully dynamic) message.
Here is a quick run-down on three shared short code services:
Textmarks (41411) - From the makers of Reactee, which brings SMS to your shirt, this is the easiest and most robust of the shared short codes I've seen. You can accept input in addition to your keyword, then act upon it. For example, HealthyToys.org created an SMS application last Christmas for parents to check toy safety from the store.
Textmarks also lets you broadcast messages to users who have signed up. You can message everyone, just one user ("Hey! There's a new match on your saved search."), or any subset of users (build a private Twitter?).
The service is free, though they've reserved a number of popular keywords. Also, response messages include an advertisement (right now for themselves).
Mozes (66937) - Their service seems to be aimed at bands, but it's useful for anyone. Just like MySpace! By default, your keyword will reply with a static message. Mozes also makes it way easy to reply with a link to download media that they host.
The documentation is good, but it's not as easy to get started as Textmarks. Plus, Mozes only gives you one keyword for free, and that will cost you $5 per month starting in 2009.
DOTGO (368266) - This service takes a little bit different approach. If you own a domain, you already have a keyword reserved: it's everything before the dot-something extension. To create content for your keyword, you need to have an XML file that describes the types of queries you expect.
Sadly, it looks like you're stuck with static replies (though more than one), so the type of application you can build is limited. Like Textmarks, DOTGO is free and ad-supported (also just house ads right now).
Update: DOTGO co-founder Stefan Gromoll tells us that the service can provide dynamic content and broadcasts to users who have subscribed. Way to go! It sounds like a full-featured SMS application builder. Gromoll points to their documentation to learn more. Also, he points out, advertising is optional (determined by the publisher—that's you).
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