For one thing, my understanding of Rackspace's business practices -- and I've only dealt with them peripherally, so I might be a bit wrong here -- is that they "manage" things like their network, and the actual server hardware, and stuff like that. So, if you want a CPU upgrade, sure, they'll do that. If you need your server rebooted, they'll do that too. But, they don't have anyone sitting there monitoring your system's performance metrics and doing your sysadmin duties for you.
The way I read it, Rackspace did do their job: they upgraded the hardware. It was up to the server admin -- not Rackspace -- to check that the software was then configured correctly.
And finally, I don't generally agree with statements of the form, "If you don't know X, you shouldn't be doing Y", but ... looking at dmesg and top are both really, really, really standard sysadmin operations. Entry level stuff, really. Sysadmin work doesn't just mean messing around with Apache's configuration; there are many more nuances, and it's likely that their system is vulnerable to problems that they don't even know about.