Hi Folks,
With the pending arrive of a new born, I've been thinking about what I can make.
I was thinking of using an arduio, temp sensor and pressure mat to start. As a display device, I will use nabaztag (http://www.nabaztag.com/en/index.html). The idea was to monitor the baby's movements and display green for slight movements moving to red the more restless he/she becomes.
My question is about pressure mats. I was looking at the Flexiforce Pressure Sensor from spark fun but had some concerns. It has a max weight of 25lb. That may be a bit high for any decent detection. Most of all, the area is so small. If I mounted it between two pieces of wood, it may work. Or may not...
Is there a better idea for a pressure mat? I need something that gives me a linear output based on force / weight.
My other option is to use a pair (x & y) of piezo vibration sensors.
How about an accelerometer? You slight movements be detectable? How much noise would there be when the thing is at rest?
Any other ideas? I want this to be the first ever internet connected crib ;D
Technically, I can report that accelerometers will indeed be noisy and probably won't function well as a baby fussiness sensor.
Non-technically, your baby will be quite vocal about letting you know when it needs something so a movement sensor will not be necessary and will quite likely elicit a great deal of eye-rolling from your wife and associated relatives.
The best thing you can do with the money you have set aside for this project is to start a college fund.
Fussiness sensor - love it.
"Setting aside the money for a college fund" is hardly in the spirit of making things.
But setting up a college fund is helping to make sure that we'll continue to have people interested in making things 
Some more things to think about... your baby probably won't be 25 pounds in total until he/she is around 12 months. You'll have a good feel of the rhythm of your baby's sleep schedule by then, too. I have a 13 month old and he's about 25 pounds.
Congrats, and welcome to the club, btw! 
You might try making a 'mat' using two thin pieces of wood and 4 of the pressure sensors. Place the sensors at each corner of the board. All the weight on the board would then be distributed across the 4 sensors. As the baby squirms around, the distribution of forces should change, and the readings from the four sensors would fluctuate. It would take some pretty fancy algorithm to figure out the pattern or forces, and then determine how much activity there is. You'd also probably have to put the board contraption under the mattress, although with 4 sensors sharing the load it might be ok.