See my company Live Documents www.live-documents.com. Built in Flex. The advantage it gives us is the same code base can be used for the web, desktop and mobile platform (yes that includes ios).
After Gdrive is out, DropBox will find it difficult to grow.
The thing about lockin might be true for existing DropBox users. But what If I am a new user and I am comparing Google and DropBox. Who do you think I will choose?
Also once google adds support to gdrive into all their existing offerings it will be difficult to compete with that.