I've always used these terms as follows:
A "soft reset" is a battery pull or equivalent, similar to rebooting the device
A "hard reset" refers to resetting the device to out-of-the-box condition.
For example, I have a windows ce based GPS device that has a soft reset button and a hard reset toggle. The soft reset reboots the device. The hard reset toggle reloads the OS from the device ROM, restoring the OS to factory condition.
The only way to do a hard reset on a blackberry is to reload an OS and NOT restore user data or completely wipe the device and reinstall the OS.