I occasionally like to show off how I can do some pretty sophisticated calculations and experiments right on my Macbook. For these tasks, over-speced machines and special purpose hardware are sometimes assumed to be the right solution, but a judicious use of the resources on a MacBook may get the job done in a similar time.
One thing that would be nice is if the OS actually supported 64 bit address spaces (after all, the Core 2 Duo processor is a 64 bit one). This comes in handy when one wants to mmap large files (e.g. greater than a few gigabytes). On 32-bit architectures, the address space is limited by the word size (in this case bits or about 4 GB of space and even less-so with kernel limitations).
Supposedly the next version of OS X will have support for this (and will come on the MacBook Pro installations, but I assume not the MacBook).