TextMarks has been on my radar for a little while. It's a company offering free (ad-supported) access to SMS shortcode 41411, via a secondary keyword of your choice. They also have pro options with no ads, and shorter (or reserved) keywords.
Not having a great idea for a SMS-based app (I think I'm biased from having web-enabled, push-email PDAs for too long), but wanting to kick the tires, I decided to build an iteration of the now-classic "to-do list" app.
Meanwhile, I've been spending a little time with the Google AppEngine. When a hosting account came due for renewal, I decided to see if I could replicate it using AppEngine. AppEngine covers all the basics, and makes it easy to stream BLOBs out of the datastore as well. But if you host files (anything over 1MB), those are not allowed in AppEngine, either as static files or as datastore objects. Too great a magnet for bandwidth or data hogs I suppose. So those files live somewhere else.
But AppEngine makes a fine place to put a small SMS-driven app (assuming you don't need the background processes or scheduled processes that AppEngine doesn't allow).
Registering a TextMark is simple. Set the URL to the one you want called when 41411 receives your keyword, and add dynamic data from the SMS into the URL request GET params using escape sequences (e.g., \0 means "the whole message after my keyword"). So your URL might look something like http://foo.appspot.com/sms?text=\0).
Go into the "Manage" tab, pick "Configuration" and turn off the various options related to "posting messages" -- these aren't necessary for your app. If you aren't planning to asynchronously send messages out to your users (hard to do with AppEngine, as mentioned above), you can also turn off the subscription options.
Now you just need a handler on AppEngine to receive those calls and do something. Whatever the handler renders will be truncated and sent back to the user as a reply SMS, so you get a semi-web-like mechanism for confirming that you're done an operation, or returning results.
Create your AppEngine app, a handler, and yaml that ties 'em together, per the tutorial. Now you can just modify the basic HelloWorld to do your bidding instead.
Here is my little memo handler -- it files memos under the phone number of the person sending the text, and forwards a list of all the memos to an email address if it receives the command "export (email address)." Don't put anything private in there, since there's no PIN/authentication (although it would be easy enough to add) ... so anyone who knows your phone number and can build a URL by hand can just ask for all your notes :)
Of course, it's easy to test out your handler with just the browser address bar, but TextMarks also provides a nice emulator so you can send messages to your handler -- and see what the response will look like with their 120 char (free account) limit and their ad tacked on. And there are a bunch of other neat things you can do, like create short term stateful contexts, where a user can just text back Y/N or a response to numbered "menu" and the messages will get to your app automagically.