pulseIn() would not work for this since your trying to measure peak to peak and not from a high to low or low to high since if you think about it what if a wheel stops on a magnet, then it would have inaccurate readings. Measuring peak to peak will get the actual correct time.
Sorry it took so long to post back.

For a more detailed information, the link is as followed ---> http://basicelectronixs.com/node/41

could you explain in more detail?

Hi everyone, I have started a site up to upload my projects. My first project is measuring speed of a wheel using a hall sensor. To view what I have so far, please visit http://basicelectronixs.com/ and navigate to electronics > projects > speed sensor. So far I have implemented one version of the code but I will be posting another version up as soon as I tweak some things out. The version I have now measures revolution over x time while the newer version will measure x revolution over a given time period. This newer version which I will try to have up also consists of a op-amp design so feel free to look around, explore and learn!  

Hi, I created a speed sensor design using a hall sensor, magnet and an arduino. I've been looking around the forums for answers but came up half handed with answers. For all those who has some trouble, I have some code up and instructions to getting a speed sensor running. The code so far has some hiccups and is kind of messy but for now it will run the basis through a serial monitor
http://basicelectronics.x10.mx/?page_id=38

Did you make any progress on this so far. I like the idea of reading voltage since it seems as if Arduino would better interpret this. Then I would create a calibration using the analog to digital to find the digital voltage coming out of the thing and calibrate it. Kinda hard to explain but message me and I will be gladly to help since I have some code up!