I've decided to build a beowulf cluster out of Raspberry PIs. I figure I will need 8 slave PIs, model A, and one Model B. They'll be connected via usb, as USB 2.0 is faster than 100BASE-T, and the model As don't have ethernet. I'm going to build this at some point in the future, to wait for the release of the model As, and for the price of HDDs to go down (they're outragously high atm). I think this will be much more feasable than a tablet, as all the hardware would be compatible as-is. When built, I'm hoping for a price of about $450. All I have to do now is rope in people with money.

USB 2.0 is not faster than 100BASE-T for networking; it's intended for host-slave communication. I suppose you could connect model As with SPI or I2C networking, but I think the extra $10 might be worth it (or cheap USB-Ethernet adapters, if you can find them under $10).

For what do you need the HDDs? The Raspberry Pi loads the OS from the SD card by default (which doesn't deny you from using a HDD for storage, or to put a special bootloader on the SD card that loads the OS from the HDD).

In my opinion, the onboard ethernet of the B model will work better out-of-the-box than a USB-Ethernet adapter. Also, the model B has more RAM (that means you need to connect less devices to have a cluster with more total RAM)

It sounds to me as if he wants to use a big HDD for storing the results(?) of whatever computation he's performing on this cluster? I guess?

Heartily agreed here. I think using Model As instead of Model Bs to save $10 is cutting off your nose to spite your face.

More than that, model B Raspberry Pis are expected to be released before model As, so you won't need to wait for model As to be released.

(Honestly I even think they could drop the model A and eventually focus on lowering even more the price of the model B, as I don't see that many people interested in model A. But that's another story.)

A definite possibility indeed, were it not for the fact that they need to make sure they continue to cater to the education community.

What are the benefits of using a multicore processor, or a cluster such as what was stated before, instead of a single core processor with similar FLOPS and clock speed?

Learning about cluster configuration and network administration.