Amazon S3 was the second most popular choice in the prioritizing special remotes poll, and since I'm not sure how I want to support phone/mp3 players, I did it first.
So I added a configurator today to easily set up an Amazon S3 repository. That was straightforward and didn't take long since git-annex already supported S3.
The hard part, of course, is key distribution. Since the webapp so far can only configure the shared encryption method, and not fullblown gpg keys, I didn't feel it would be secure to store the S3 keys in the git repository. Anyone with access to that git repo would have full access to S3 ... just not acceptable. Instead, the webapp stores the keys in a 600 mode file locally, and they're not distributed at all.
When the same S3 repository is enabled on another computer, it prompts for keys then too. I did add a hint about using the IAM Management Console in this case -- it should be possible to set up users in IAM who can only access a single bucket, although I have not tried to set that up.
Also, more work on the standalone OSX app.
I think a lot of people misinterpreted "phone/mp3 player" as "Android"; I know I did.
As you already support directories, MTP would be needed to support the other half of pretty much all phones and media players (other than those made by Apple). MTP is a valley of pain, though.
You can easily do that with IAM, checkout http://andrewhitchcock.org/?post=325.
It boils down to creating a policy for each user/bucket. But allow read access to the full bucket list (otherwise the AWS Console won't work).