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: 
You should use an ARM. Then you can run full linux or android and have access to powerful libraries, high-level functional languages, and a package manager and community. You can use gcc or LLVM, and a modern debugger like gdb.

ARMs used to be too expensive and/or too big, but nowadays you can get an ARM for $5 that's only 13x13 mm. You have to use reflow soldering, but you will anyways if you want to make a professional-quality robot.

<http://www.eetimes.com/electronics-products/electronic-product-reviews/processors/4230227/TI-debuts--5-Sitara-AM335x-ARM-processors>

All other instructions sets have lost the competition. If you pick something like AVR, you will be forever stuck with inferior toolchains, weaker MIPS/dollar, and a much smaller community.

If you don't want to engineer the whole motherboard, then Gumstix, BeagleBone, BeagleBoard, and Raspberry Pi are all excellent pre-existing ARM-based devkits, and processor vendors also offer a devkit for every processor they make, bringing out at least a display bus and some serial busses.
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.