i just had a look using the gconf editor, and found that there is a box under /system/osso/af labeled keyboard-attached. this becomes checked when i pair my bluetooth keyboard with my N800, and should in theory go unchecked when i turn the keyboard off.

turning the check on manually seems to stop the onscreen keyboard from going active.

so people, if you have trouble getting the onscreen keyboard to show, go and see if the box is checked, and if it is, try to uncheck it.