I am the developer of a database modeling tool in which we use an interactive real-time force directed graph layout algorithm. Recently I came across the Gephi application and thought that perhaps our work may be of interest. In order to achieve decent performance (the application is written in HTML5 and JavaScript) we had to use a simplified verlet physics to calculate the attracting and repelling forces, the manhattan metric for measuring distances, introduce a "radius" (actually square) of influence, and handle the stopping/starting velocity on individual nodes. We use quadratic curves for the edges, and the edges have a stiffness, i. e. they strive for straightness. Technicalities aside, the resulting graphs look quite nice and the level of interactivity is high.
Take a look at the application here: http://www.anchormodeling.com/modeler (using an HTML5 capable browser, like Chrome, Firefox, or Safari)
Source code is here: http://code.google.com/p/anchormodeler
I can explain the code further, should you be interested to incorporate similar functionality in Gephi.
Regards,
Lars