Is it possible to force the user to upgrade once there is a new version of my application available in the iTunes Store?
I am presently working on an application. However, I want to force the upgrade whenever I upload a newer version, because the upgrade will have more features and I want to discard the previous version. Is this possible by default with iPhone or do I have to write my own implementation to check the current version and compare it to the store?
The only way system handles updates of your application is prompting user that update is available in standard AppStore application.
Also, standalone application is not aware of available updates so you'll need to implement your web service where application can check if the current version is valid.
And I'm not sure if this conforms with Apple rules on applications, like (can't find proof-link though) you cannot upload trial versions for an application.
* EDIT * This answer is no longer true. See other upvoted answers for current truth.