Leigh Nunan and I have made an interactive talking Portal Turret plush toy. It has a motion sensor, a lift sensor, and a tilt sensor so that it can react just like it does in the game (except for shooting at you). It's powered by an Arduino and an Adafruit Wave Shield. Pictures, video, and details (including source code) are available at the project page:
Nice project !
Would you recommend the Adafruit Wave Shield ? I was considering to use it.
I nearly submitted this to Hackaday but someone got there before me 
(Was it Osgeld?)
Depends what you want to do audio wise changes what audio shield you should get.
I'd say the wave shield is good for low-ish quality sound effects for projects and general noises. It can play music but it's not really designed for that.
For music there is the SD card shield/board from sparkfun or for high quality audio and a whole raft of other features there is the rMP3 from Rogue Robotics.
There are other boards though so decide what you want it for and then ask the question again
It will be a sort of talking alarm clock but sound quality has to be reasonably good, good enough so that you may think there is actually a person in the room. I'll use an external amplifier instead of the loudspeaker.
Well I'm not sure of the quality of the wave shield when used through an external amplifier - as speech is mainly higher frequencies (although without bass it can sound very flat and recorded) it may be OK.
Mowcius waits for someone else's opinion now
I really liked the Wave Shield, and would definitely recommend it for applications like this. I haven't tried adding an external amp, although I will be soon, because the sound is a little quieter than I'd like (it's great if you're in a quiet room, but as soon as there's background noise it's hard to hear).
For more sophisticated uses, I also recommend the rMP3 from Rogue Robotics. More expensive, but much more fully-featured.
XD
Now, make it sing, and hide it behind a grate in a wall. (Coincidentally, I found that easter egg on Easter!)
Antimov competition version:
Incorporates a light sensor, when you aim a laser pointer at it (or a real high power thermal discouragement beam), it sets itself on fire and explodes...
Not before screaming of course.
Portal 2 is so epic!