[tptacek]

Does it make a lot of sense to build a Riak cluster in a virtualized environment?

The clear win for Riak that I can see is being able to easily replicate some of the characteristics of AWS on in-house hardware.

[siculars]

AFAIK, the official stance from Basho is to not use a virtual environment. That said, Joyent does officially support Riak in their environment. The thing to look out for is not slow i/o but variable, unpredictable i/o which is what you get with virtual machines... unless they are your virtual machines in a private cloud.