I was looking at thier sdk a few months back, they only require that it fits thier api ( naturally ), and they have extremely tight testing before they will accept the plugin module as being acceptable for inclusion.
thier servers are red hat linux, and the cms is a lamp / lapp* app. :)
I have been a "member" of thier flagship site for longer than I have been a member here.
it wasn't so much a looking to do business with as a what about the concept of paying thier fees, and not having complete control over what could contain sensitive data for your business.
with all the time I have been a member of renderosity, there has never been a serious hacking attempt that was sucessfull, they actually do very well at monitoring for unwanted activities and will take legal action to stop it, before the person can do any damage.
it doesn't scale well, once you are doing 4 terabytes of data transfer a month, but it can handle up to that quite well.
*it may be postgresql rather than mysql. I have never probed to find out.