Hey ien,
I believe DO’s apt-get repo’s are just caching mirrors of the official Ubuntu repos. Version 1.1.0-beta3 is the most recent version that is in Ubuntu’s repo (which usually lag a bit behind current releases). You can check which version is available (and what repo it comes from) with the command sudo apt-cache policy s3cmd. For example, from pure Ubuntu server outside of DO:

$ sudo apt-cache policy s3cmd
s3cmd:
  Installed: (none)
  Candidate: 1.1.0~beta3-2
  Version table:
     1.1.0~beta3-2 0
        500 http://ca.archive.ubuntu.com/ubuntu/ trusty/universe amd64 Packages

And from a DO droplet

$ sudo apt-cache policy s3cmd
s3cmd:
  Installed: (none)
  Candidate: 1.1.0~beta3-2
  Version table:
     1.1.0~beta3-2 0
        500 http://mirrors.digitalocean.com/ubuntu/ trusty/universe amd64 Packages

The s3cmd tools site itself says that the repo’s are not up to date, and for the latest version you need to either download it from sourceforge or github.

http://s3tools.org/download

Hope this helps!