I give two out of three a little mention here -
I plan on compiling and running some of the code from the rcarduino blog on the teensy over the next week just to confirm portability but nothing that I do really stretches an UNO never mind a Due or Teensy.
For stretching the hardware you should try DIY Drones, thats one area where the size of the teensy will be a major advantage.
EDIT: The main reason I am interested in the Teensy and Due is for audio where speed and 32 bits will have a big impact. The teensy looks interesting for the DSP instructions which may make software filters a possible addition to Arduino audio projects.
Duane B