Question: I am looking for a starting point for my project, preferably using popular systems (ones there is a lot of support for). I have an Arduino Uno, a Raspberry Pi, and a lot of willpower :) Anyone here built a project using the systems above?

Observation: I'd like to start with a simple line-following vehicle and build up afterwards.

Answer: 
If you want a line-following robot, then something similar to an [m3pi](http://mbed.org/cookbook/m3pi) would be achievable. Photo-transistors seem to be very effective with a black-on-white track.

As for a microcontroller, Mario Markarian is probably right, it is down to personal preference and the project you are working on. The m3pi uses an [mbed](http://mbed.org/handbook/mbed-Microcontrollers) and has a lot of IO's to play with. For more advanced robots a raspberry pi or [beagleboard](http://mbed.org/handbook/mbed-Microcontrollers) work, interfaced with slave microcontroller(s) to provide sensor data and offloading any repetitive computations that could otherwise slow down the pi.
If you want to go further than using microcontrollers you could run ROS on you [Raspberry Pi](http://www.ros.org/wiki/ROSberryPi/Setting%20up%20ROS%20on%20RaspberryPi).

I have build several robots and a couple of other projects using a [RoboCard](http://robocard.dk/) (site in Danish, but can [be translated](http://translate.google.com/translate?js=y&prev=_t&hl=en&ie=UTF-8&layout=1&eotf=1&u=http://robocard.dk/pages/home.php&sl=da&tl=en)). The RoboCard is build around an ATMega, so that certainly is a viable route.