If you're just looking to deploy a single application on Digital Ocean or some other VPS, the digitalocean plugin for knife will really help out (https://github.com/rmoriz/knife-digital_ocean).
EDIT: In regards to Stackato...
-- (1) command-line client for deployment via git repository and doing anything related to deployment (add new instances, remove instances, create new environments, assign domains to the PaaS router, etc.)
We have a CLI for doing this, as well as a web API and a Python library for communicating with the API (see https://github.com/bacongobbler/python-stackato)
-- (2) clear pricing
We have a clear memory-based pricing plan. This allows users to scale out their infrastructure to as many nodes as they like, and only have to manage their total memory in their clusters to figure out pricing.
-- (3) I can pay for more memory as I need it
It's on your own hardware. EC2/Rackspace/VMWare pricing depends on their model.
-- (4) ability to specify environment variables in a configuration file that take effect on the container process
We have a couple of default env vars in the application container, as well as letting users configure their own envvars. See http://docs.stackato.com/reference/environment.html and http://docs.stackato.com/reference/stackatoyml.html
-- (5) web interface for administering the account
We have both the CLI, the API, and a web interface. Check out our YouTube channel at http://www.youtube.com/user/ActiveStateSoftware for tutorials around the Stackato console, or just take a look at http://docs.stackato.com/ :)
Disclaimer: I work as a product manager for ActiveState, working on Stackato. We'd love to have you over here! We're also on Freenode, channel #stackato if anyone is interested in this stuff :)