In the last few days I was working with Arduino and the Wifly Shield ( http://www.sparkfun.com/products/9954 ), it's pretty easy to use and there is a good library.
You will find the library on the Arduino forum but this will not work on Arduino 1.0, I am so generous to share with you the library that works. Click here to download.
I really suggest you to buy this shield if you want to put a wifi into your project because you can also doing some stuffs like: ad-hoc wifi, sleep mode,etc...
The Ad-hoc wifi is something that I really like it because you will able to switch your Arduino into an access point , just think about if you want to create a car that you can control it by an Android or Iphone, yes you can use a bluetooth device but the range is limited.
Enjoy your wifi and make something cool.