If you are doing a large block device write operation such as using dd to write a 2GB Raspbian image to an SD card for your Raspberry Pi then you might see your system load go high (I've seen 6 or higher for a slow SD card).
The reason is due to a kernel IO scheduler bug that is fixed in kernel 2.6.36
If you run
watch cat /proc/meminfo
you will see MemFree falling as Cached rises because the device you are writing to cannot write quickly enough so the OS queues up the blocks to be written in the disk cache (which uses any unused RAM).
Updating your kernel to 2.6.36 fixes this IO scheduling issue.