When iCloud came out, there were some comparisons between it and DropBox. In a way, both are similar. Both are cloud storage services for users. In contrast to cloud storage services for developers, developers no longer have to provide their own cloud storage. Users provide their own storage. If users needs more, they buy it.
iCloud right from the start is a framework for allowing developers to store their apps’ data on users’ iCloud storage. For now it’s Apple who has those apps, but any app developer should be able to use iCloud.
DropBox now they has an API allowing developers to store their apps’ data on users’ DropBox storage. But they, started out as cloud storage for non-cloud apps. You store files such as Word and Excel files online and you can access it in any of your devices. They’ve been pretty successful with this feature.
So currently, DropBox offers more. But an enterprising developer, maybe even Apple, could easily develop an app cloning DropBox’s cloud storage for non-cloud apps feature using the iCloud as framework.
Is there a threat to DropBox? Definitely. As cloud storage becomes more important, there will be a consolidation and/or standardization as applications support just one or two services. And a huge user base plays a critical role here. If they can’t rapidly increase their user base, they need to partner with someone who has one.
iCloud of course has an instant huge user base thanks to the iTunes store. But DropBox has already famously declined Apple. The other huge user bases I can think of are Google, Amazon, and Facebook. Google and Amazon are currently executing their own cloud strategies with Google rumored to be coming out with cloud storage for users (probably a repackaging of Google Docs) soon. Facebook?