Update: Thanks to Ryan for pointing this out. You can actually have Drupal multisite run on Mosso, but you will need to create sub-domains and point your other domains to these subdomains at the DNS level. For example, if you site is 123.com and your other sites are 245.com and 678.com, you will need to create a.123.com and b.123.com. Then you will point 245.com to a.123.com and b.123.com. You cannot just point all domains to the server root as you would.
About 3 weeks ago, we moved a high traffic Drupal site: www.thewrap.com to the Mosso cloud hosting environment. The site had over 250,000 unique visits last month and being a publishing site is really content rich. Before the site was moved to Mosso, it was hosted on 2 Rackspace servers.
As the traffic volume increased, we started looking for a more scalable hosting solution. One of the options was to get more servers for the stack and the other one was to go with a cloud hosting solution. This is our first experience with cloud hosting and we were initially worried that the scalability that is claimed may not be there.