So, to put a final update to my trials with Rackspace Cloud Sites (specifically sites).
Right now I have 2 or 3 sites that flat out won't run on MODx, no idea why... just throws odd permissions errors, wayfinder won't complete, odd things. We have over 100 other sites working just fine.
We fired up a cloud server, moved the odd behaving sites over to it. They run perfect.... no better than perfect, they run FAST! Not quite MODx Cloud fast, but almost.
We're in the process if migrating off. I still like RSC as a service, but the sites offering just won't hold MODx anymore, sad because in 2008 when I joined, then Mosso, it was amazing and fast. I suppose it was inevitable.
Also if anyone is curious we've been playing with this
http://www.openpanel.com/ as a panel, I like it. Has some of the simplicity of the UI that I liked on Rackspace Cloud Sites.