What is the best method, if there is one, of using Wordpress as a CMS for users to input data while not actually building the website on the Wordpress platform?
One possible solution I've found is using a PHP class called "WPGet" (http://peter.upfold.org.uk/projects/wpget) which fetches directly from the database.
You could use Wordpress' API calls, but depending where your "custom built application" sits, it might require including numerous files in your code (not very fun/efficient)
I have a site that requires constant updating. It's a golf club site: results, news etc all updated via wordpress by the club, or individuals with interest (juniors, ladies, seniors each have a mini-site) - and not always by me - so i have a front end, the standard site with static .php pages, which i then feed data to, by calling on posts marked in categories from wordpress, relevent to the site's page. The coding is easy and it's a quick and simple way of using wordpress as a cms, but it does strip away most of the Wordprss functionality, eg. comments, But I even use it for galleries with lightbox.
Like i say - it's nothing major, but sounds similar to what you're asking...?