What DMBS do you use with Erlang ? and Why ?
Here is an evaluation of DBMS in Erlang:
http://www.erlang-solutions.com/thesis/dbms_eval.html
UPDATE: Mind it's a bit out-of-date, but it could turn useful as a start.
Mnesia, because it's all native erlang, and being able to store erlang terms is just great. Speeds up development too.
I know of no other DMBS that can integrate so closely with the language (that one's easy, Mnesia is almost part of the language, a Dict on steroids). Of course, it's a bit low-level, so functions like full-text search have to be coded against it, but it wouldn't be that much fun otherwise (unless you can couple it with some other open-source full-text search engines).
But you have to judge the different options for yourself against your own requirements.
I would use postgresql with the same code as this project. http://zotonic.com/ (just surf around for a sec, feel the low latency, I love it)
Postgresql will always be a little bit more mature than mysql. =)
It worth looking into. (Have not used it yet, but I will)
I have heard that Mnesia with Tokyo Cabinet is a powerful and popular combination. Tokyo Cabinet replaces dets (which has an 2GB limit). I have been looking for how to set up them together and found mnesiaex and tcerl. I haven't testet them yet (but I will), and I don't know how production ready and reliable they are. But I have heard that one finance company use the combination of Mnesia and Tokyo Cabinet. If you google it, you will find some talks about it.
CouchDB is also popular, but I don't know if it has any advantages over Mnesia when it is used from an Erlang-system.
UPDATE
I have reasently come across Riak when I was looking for a scalable database that I could use instead of a traditional RDBMS. Riak is inspired by Amazon Dynamo and has the characteristics of high-availability, tolerant to network partitions and eventual consistency. It looks really good and I will try to use it in my coming project.