I think it would be a very good idea to expose some kind of API for working with arcgis.com
For instance, right now esri products/clients have the ability to search arcgis.com for data or web maps.
However, if one develops a custom application with any of the API's this option is not available.
Or perhaps add new content to arcgis.com
The only option available now is adding an already known item to a map.
I believe this would also motivate people to further use arcgis.com as a platform for publishing and using public data.