Basho's products look great, but I hardly hear of anyone using them. Most folks in the Ruby community tend gravitate towards MySQL, MongoDB, and Postgres. Any insight as to why that is? Riak looks like a pretty solid database too.
http://devblog.seomoz.org/2011/10/using-riak-for-ranking-col...
I know of a couple other ruby shops that use riak. Github uses it a bit. Spreedly uses it as well.
Postgres is a better fit for most Ruby web applications. When you first launch your app, you'll have a tiny handful of users and won't have any problems fitting it in a free Heroku instance on the free 5MB shared Postgres, and the compromises you'd have to make with your data model to fit a database like Riak aren't going to pay off until you get enormous. Postgres and ActiveRecord get the job done quickly and without fuss, and allow you to launch quickly and see how your app is received.
You can always scale later: https://gettingreal.37signals.com/ch04_Scale_Later.php
Parts of it may well be open sourced over time.
Riak has a tunable consistency model, and there are a lot less applications that need tunable consistency compared with those that need the strengths of the SQL databases (everyone knows them, they just work) or MongoDB (designed from the ground up to be a developer friendly document database).
The closest Riak competitor is probably Cassandra (in terms of technical attributes), and you don't see huge numbers of Ruby apps using Cassandra either.
http://wiki.basho.com/Riak-Comparisons.html is worth reading.