Multi tier architecture implementation on Python
You should checkout the Turbogears framework as it is composed of several popular components: ORM with sqlalchemy, pylons for logic and support for WSGI, permits support for several templating engines for the frontend... endless.
I use it for several web-services behind AJAX-enabled front-ends (like Flex-based apps, among others). You can front end the TG2-based webapp with apache or your favorite WSGI-enabled web server too.
Checkout their website since they have a tutorial to setup a wiki in 20 minutes.
Cheers!
I need to create web application, which can be reached by user as regular web site and as XML-RPC web service. Also web site should have mobile version. I'm planning to use next technologies:

Django (for web frontends (regular and mobile)).
Pyramid (for web service).
SQLAlchemy, Memcached (for persistence level)

Later other projects can reach this data and providing logic, so I think it is better to make two tiers. I see it in next way:

Tier 1. Main logic service level. This level will provide API for frontend applications (Django powered web site, for example).
Tier 2. Different mostly end client applications (web site, API for remote client devices).  

For communication between this tiers I'm planning to use XML-RPC protocol. 
In this case it will be easy to scale it and add new front end application or connect another projects to this (I believe it). 
I have main question, -- what can I use to make it easy build first tier? Maybe there is some framework good for that?
And what do you think about this whole architecture. Because I'm filling that I'm thinking in Java terms developing in Python. Maybe there is some another idioms in Python world for such situations.
Thanks for you time and help.
P. S.
Some links for reading are welcome.