The toon effects in "A Scanner Darkly" was done with digital rotoscoping, in which they basically took live-action footage and applied a toon filter to it in a high-end proprietary video FX software.  It's like applying filters to a movie in Adobe Premiere or Autodesk Combustion.

The director of the movie talks about the process here:
http://www.highend3d.com/news/film/165.html

As for the original question, I would recommend saving up your pennies and investing in 3dsmax with either FinalToon or David Goulde's Illustrate plugin.  MentalRay also comes bundled with Max, and the toon shaders in that renderer have been used many times in production, so they must be good.  Same with Maya or XSI (they both use MentalRay too).

Otherwise, I'd go with Carrara.  Not as good, but better than Poser.