Chances are if you’ve found this post, you’ve been googling around to see if it’s possible to run a WordPress multisite installation on the Rackspace Cloud Sites. Obviously if it’s a sub-folder install you can as there is nothing tricky to that. (this is where you’re sites run in a folder like domain.com/site1/)
Where it can be harder is when you want to have users with fully qualified domain names each pointing to their own site, within your multisite install.
In short: Yes it can be done.
Primarily I’m posting this to dispel a lot of what I’ve read on many forums saying that it can’t be done. The trick to doing it us WPMU’s domain mapping plugin.
A bit of background. Usually you need an environment where the main hostname is glued to a dedicated IP address. The test suggested in most places is visiting the IP address (e.g. http://126.96.36.199) by itself and you should get to the main site. What this is demonstrating is that Apache is configured correctly to support multisite.
You won’t get this kind of setup to work on Rackspace cloud sites due to the fancy way they have their environment setup, even if you order a dedicated IP address.
The trick is to firstly configure WordPress for a sub-folder install. Test that and confirm that it’s working.
Then use the WPMU domain mapping plugin (located here), walk through the setup of that plugin.
Final piece of the puzzle is to create each site, then in the Rackspace Cloud administration you have to set up the domain name that you are mapping as an alias of the main domain name. Sure the catch here is that you have to have the DNS pointing to their DNS servers but if you can live with that, it works.
It looks like this in the control panel when you’re done. (click to view larger)
Good luck! Post a question here if you have one, I’ll help out if I can.