So is it not in fact up for 24hrs if it has to wake up when you try to access it again?
However you can work around it by hitting your server with an http request every 24 hours, or upgrade to the Bronze Plan which apparently has $0 base price anyway so I'm not sure what the downside is.
Also, for a relative novice, how easy is it to get a Rails or Sinatra app up and running?
My concern longer-term is that Openshift would go the same route as Heroku. Is there anything about their business model that might indicate that's not a major risk?