Google Drive, the cloud storage service rumored to be launching soon, looks like it will include support for third-party apps and a software development kit. Google Operating System has looked at the source code for Google Docs and found several mentions of an SDK in conjunction with Google Drive, including a mention of "google-apps.drive-sdk" and an SDK "open with" command. This isn't conclusive proof, but it suggests that you'll be able to open Google Drive files with other applications.
There's also apparently a message suggesting more integration between Drive and Gmail: "Say goodbye [to] email attachments and hello to real time collaboration. Drag anything shared with you to My Drive for easy access." We'll know more in the coming weeks; for now, here's an image of the relevant code.