I'm currently in the process of building one now, using an Uno and the WiShield. I have 4 Maverick ET-732's for the probes and a 20x4 LCD for display. I have it all breadboard'ed out just working on laying out my prototype shield with my connections. I have an Electric smoker so my controller just runs a 110v SSR to control temps on the oven based around a set point (basic PID). It's a really simple project but will get me much more stable temps. AdaFruit recently ran a tutorial
on setting up a Thermistor. The way they did it was much easier than my initial way, which was to calculate the A,B,C values for the 732's and run them through the Steinhart-Hart equation, they also used AREF and +3 volt to stabilize the readings which helped a lot. So I'm in the process of rewiring to take advantage of those changes. Initially I got side tracked trying to use a 'cheap' Wireless solution, eventually gave up on that and just went w/ the WiShield. It works pretty well but the Uno doesn't have much power so my current software is uploading the temps to an EC2 instance then my android app pulls from the EC2, also I have a simple web page on the EC2 that I can use to check on and adj the temps. This way the Uno only has to communicate with 1 external server and any number of clients can attach to the EC2 instance w/o slowing down the Uno. IMHO this is where the CyberQ WiFi screwed up, 3-4 clients connect to it, it almost becomes unresponsive, not very scalable.
twinsfan, you'll probably have to use a relay or transistor as mentioned and power the fan using some other source than the Arduino board. So, if you are going to power Arduino using a 12v transformer just split the output on the transformer run 1/2 the Arduino and the other 1/2 to the transistor/relay then 'pulse' the transistor/relay using the digital out to get the desired fan output like 25%, 50%, 75%, etc something other than full on/off, that's basically what I'm doing w/ the SSR in my project, since the heating element is on/off I turn it on for 10 seconds, turn off for 10 seconds, repeat, that's 50% power, or on for 15 seconds off for 5, that's 75% power, you just have to play w/ the times and get it tuned up for your needs.
If you are interested I have some pictures of the 'in progress' here,