I've installed FEnICS into SageMathCloud systemwide, for anybody to use. It should be available from Python, IPython, Sage Worksheets, etc., via the usual "from dolfin import *", and should get updated regularly. Notes:
if you use Sage worksheets, you'll need to put %python at the top of the cell, or be careful to coerce floating point literals to floats, or disable the preparser.
There is of course no support for FEnICS's plotting of any kind, since SageMathCloud is a web app, not a local desktop app. That said, this could improve if somebody were interested, e.g., a FEnICS developer... And of course both Sage and IPython have plotting capabilities, which can be used with the raw FEnICS data.
(note -- click "Restart Project Server" in project settings if you're trying to use FEnICS through a Sage worksheet and it isn't there yet.)