Currently I have a bunch of servers scattered across a few different providers e.g. EC2, rackspace. Some boxes are Linux and some are Windows. Currently I'm finding the administration of the firewall rules to be quite labor intensive. Are there any solutions out there that provide a centralized management platform?
I believe you are looking for Security as a Service providers.
The idea is to route all the traffic through the service provider and the service provider will inspect the traffic and send a decision to the server.
I did a quick search and found the link below. http://www.virtualizationpractice.com/security-as-a-service-is-it-safe-9929/
The link talks about 2 different providers(read the first part about cloudpassage which i believe is what you are looking for)
Cloudpassage runs a daemon on the servers and this daemon will contact the providers grid (over an encrypted channel) and all the security processes are run by the grid
The first link says that cloudpassage supports EC2 and rackspace