tfoote:
what is the feasibility of putting together a supercomputer using arduino? what all could you do with say 20 spare arduino megas with Ethernet. what are the limits?
Depends on what you mean by 'computer.'
Related to the reference to the 64-Rpi super-pi, Rpi and Arduino are completely different animals, with different purposes. Rpi has all sorts of conventional computer I/O, but no individual I/O control pins the way Arduino 'sees' them.
Given that distinction, Ardiuno is in not a computer that way an Rpi is, it is a controller.
Yes, you could build a super-microcontroller. It's NOT easy to address 1000 discrete I/O interface pins in Arduino, even with a whole boardfull of shift registers. However 20 Mega2560s can instantly address those 1000 I/O pins directly with no external hardware, with one 'supervisor' Mega could manage the array of the rest of them.
The super-pi has 64 Rpis. 64 mega2560s yields about 3200 I/O pins. Very cool, if you need them. 
I don't know how fast the supervisor Mega can deal with those pins though. Whether ethernet (with a shelf full of routers,) or I2C or USB makes more common/$ sense, I leave as an exercise for the student -- who would be -you- BTW since it was -your- idea. 