It also depends on what you are going for. Do you want to use the arduino or are you going to pull the chip and use it standalone? This is a critical factor. If you are going to use the arduino then that sparkfun box is the best possible solution because it is designed specifically to fit the arduino.
If you are going to use a chip only version, then you have LOTS of options. My three favorite are:
http://www.okwenclosures.com/ - They have a grat selection and will sell one offs. If you are buying for personal use, they actually cal to get credit card info which was kind of weird but I placed an order at 2:00 AM and had a call before noon the next day. Their cases are very good quality. I have several of their MiniTec cases as well as two sizes of their soft case. They also have a great range of accessories that work with a variety of their cases.
http://www.boxenclosures.com/category/category.html - They have a decent selection and will sell as one offs as well. I was not as impressed with the quality of their cases but they are very easy to deal with. They also have weird ideas of battery compartment sizes that don't always fit with hobby electroics.
http://www.polycase.com/ - These guys have a great selection and their quality is up there with OKW. Their VM series handheld is incredible. I have purchased two different sizes of these with the poly sleeve and could not have asked for a better product for the price.
The disadvantage of any of these or CrossRoads' solution is that you have to desgin your own PCB. However, if you are using a standalone 'Arduino', you will liekly be doign that anyway.