We used Drupal to create a mobile Application in under a month !

www.zipply.net is a simple and addictive service to convey your life experiences and emotions using your mobile phone camera and to plug them into your social networks.
It currently works on Google Android G1 & G2 phones and we've submitted it to an Android Development Contest held by a mobile operator in France (SFR)

Drupal is being used to serve the classic web pages at zipply.net but also JSON requests and mobile web pages for the mobile phone. It took just a little research to find the perfect modules for this and we've been able to leverage all the usual modules to feed content into the application's WebViews.

We are looking for beta-testers to test the service and provide some feedback on usabililty. To learn more about us, please check us out at http://www.zipply.net

Here are the modules we used :
adsense extra_voting_forms friendlist imageapi location popups unique_avatar
advanced_help fbconnect geo imagecache maxlength services user_karma
avatar_blocks filefield geonames imagecache_actions mobile_tools svn-commit.tmp~ views
cck fivestar gmap imagecache_profiles mollom taxonomy_image views_galleria
custom_service flag gmap_addons imagefield nodewords token votingapi
devel flag_abuse google_analytics json_server pathauto twitter xmlsitemap