How limiting are web frameworks
I have used Ruby/Rails for years now, and unlike just about every other language/framework I have used (across nearly 15 years of Java, PHP, ColdFusion, ASP, etc etc) it gets out of the way when you need it to.
It sounds like you might benefit from a "lighter-weight" framework like Sinatra, but with the upcoming Rails 3 release the benefits are becoming less pronounced. Rails 3 makes everything configurable ... in fact, Rails will now just be a particular set of plugins and extensions sitting onto of an infinitely flexible core. 
I am interested in this statement:

"The service will rely on a database however for a few of the more important operations
  there is no clear way of persisting a "model" object directly into a database table."

Not sure what you mean by this statement ... at some point you have something going into the database, right?
In most non-trivial applications you rarely have a single model tied to the end of a request ... you might actually have a quite complex network of models that are returned or updated.
If you are working with JSON, I would definitely suggest looking at a database like MongoDB. MongoDB is based entirely on storing JSON data, and may therefore fit really neatly with your application.
This is a general question about how limiting are web development frameworks such as Django and ruby-on-rails.
I am planning on building a RESTful web service which will have a purely JSON/XML interface, no GUI. The service will rely on a database however for a few of the more important operations there is no clear way of persisting a "model" object directly into a database table. In addition I require full control over when and how the data is being written to the database. I will need to maintain multiple database connections in order to use some connections only for reads and others only for writes.
I've looked at the "full" MVC frameworks such as Django and more basic ones such web.py and pylons. The impression I currently have is that if I go with the full framework initially things will go faster but eventually I will get stuck because I will be limited by the framework in what I can do. If I go with a more basic framework it will take much longer to get everything running but I will be free to do what I need.
This is what it seems like but I suspect that it might be an incorrect impression given how many sites are written in Django and Rails. Could you please provide your opinion. Am I totally wrong and there is a way to easily do anything with a framework like Django or Rails or given my requirements I should go with something like web.py?
Thank you!