[Startup] Java Backend and Rails Frontend

I have a startup considering building a Java backend and a Rails
frontend. The Java backend will take care of creating a caching layer
for the database and offer other additional services. The Rails
frontend will mostly be for creating the webapp and monitoring tools.

What startups/companies out there are using this kind of setup? What
are some gotchas in terms of development speed, deployment,
scalability, and integration?

(What would be helpful for me is personal experience or informal case
studies. I'd like to de-priorities answers addressing alternatives
like Grails or JRuby unless it turns out to be a big part of the
equation)

Thanks!

P.S. I cross posted this on http://stackoverflow.com/questions/4035999/startup-java-backend-and-rails-frontend.
My intention here though is to capture a different audience and get
more of the story on the Rails side -- which is the side I'm
personally working on.

Ian wrote in post #957608:

I have a startup considering building a Java backend and a Rails
frontend. The Java backend will take care of creating a caching layer
for the database and offer other additional services. The Rails
frontend will mostly be for creating the webapp and monitoring tools.

I don't understand why you need a Java backend in this case. Can you
explain further? At the moment, it sounds like what you describe as
"backend" should in fact be split between the DB server and the Rails
app.

Best,