It seemed you want to do several distinct tasks:
1) Launch and check-in on EC2 instances - seems to be the domain of the
AWS cloudwatch and autoscale services. Maybe you can say how & why
those do not fit your needs? You might look at Ylastic too though
they probably won't work for the same reasons that RightScale won't - but you don't say what those reasons are...
2) You mention Nimbus and OpenNebula, but I think Eucalyptus (http://open.eucalyptus.com/) is what
you are after? I found the "Grid" founded projects tended to miss
the mark quite badly.
3) For configuration of the machines I think you'll want to look at
[Chef][2] and Puppet. You'll want to ask a separate question about
(resource ordering) differences between these two.
