Well, when I looked inside the Arduino packaging, I noticed 4 doted lines.
I Have cut the packaging along this doted lines, and Voila:
This is the Arduino Ethernet Version, I don't know about the other Arduino Packagings.
WATCH IT !!!, I DIDN'T TEST HEAT EMISSION BY THE ARDUINO. => Autoignition point paper: 218°-246°C (424-474°F)
If you have a silicon component much over 125c, it's pretty well done, anyways.
I wonder if the folks at Arduino have put those doted lines on perpose. If they did, they should put some more. Title circles would be nice (for those screws and distance holders). And lines at the other side for SD input.
Nice find.
I have used the smaller Really Useful Boxes (from Staples, Office Max) for putting stuff in. The clear plastic is easy to drill holes in or cut out openings.
I drill a series of small holes and finish the opening with a box cutter, or use a Unibit type of drill.
You can see some of them at my link below.
Got my Arduino Eternet running for 7 days now (with working webpage) in this little box.
Still feels normal (warm but certainly not hot) 8)
missing a link to the little box?
Really cheap! 100% reusing packaging is cool! I once used my multimeter box for an enclosure:
You can glue small breadboards on top for prototyping but on a second thought, you have small children and resistors look like candies 
@CrossRoads hèhè, It isn't connected to the net. It serves an intranet page for my CastDuino Project (more on that later)
.:oomlout:. even has made a pdf file you can print and fold into a project box XD
I could upload the PDF here but let the site owner get the credit, so i'll give a link to his webpage:
Nice work... That I call recycle... I notice in the last picture, a box with a lots of leds ? Some kind of led's matrix ? That look an interesting way to place them into a small cardboard box.
Ponoko has free cardboard for designs thru the end of the month, maybe come up with an interesting box there.
It's small saving 
The part is only a dollar or two when it's not free. When I ordered 180mm*180mm jobs on ponoko, shipping is $10 and parts+work is another $10 for acrylic.