technology recommendation for LAN Dashboard
You could also take a look at ExtJS for the frontend. I've made an ExtJS frontend for a company Dashboard, and using a Django backend managing the URL dispatching, the ORM and the data retrieval (communicating with the frontend with JSON webservices) and users love it, because it's almost as interactive as a local application (use something modern like Firefox 3.5, Chrome, Safari 4 or Explorer 8 for better javascript performance) but easy to manage for programmers and administrators (no installations, no local backups, no upgrade problems, etc.)
I'm about to start a fairly large project for a mid-sized business
with a lot of integration with other systems (POS, accounting,
website, inventory, purchasing, etc.) The purpose of the system is to
try to reduce current data siloing and give employees role-based
access to the specific data entry and reports they need, as well as to
replace some manual and redundant business processes. The system needs
to be cross-platform (Windows/Linux), open source and is primarily for
LAN use.
My experience is mostly PHP/web/app development, but I have developed
a few LAN apps using Java/Servoy (like Filemaker).  I found Servoy to be very rapid and to easily make use of different data providers (DB products), but it's not open source, and any non-standard development is in Java/Swing (which is verbose and takes a lot of time).  
I'm interested in learning Python/Django or Ruby/Rails - but I'm not sure if these are the best solutions for building a mission critical data entry/reporting LAN app.  Is a web client/server really a good choice for this type of application?  
Thanks in advance for any tips/ advice.