Using Disk Druid on a system with only one drive (hda in my case):

Click "Add" to add a new partition.

Use ONLY keyboard to fill out the dialog box:
   Mount Point:       /usr<tab>
   Size (Megs):       2048<tab><tab><tab>
   Allowable Drives:  <space><tab>
   OK                 <space>
(Note how the space bar deselects the only Allowable Drive)

Error dialog:
   There are currently unallocated partition(s)...
   /usr               Allocation Succeeded
(Note how the /usr partition shows up in red.)

Highlight /usr and click "Edit" to edit the partition.

Dialog shows:
   Allocation Status: Failed
   Failure Reason:    Allocation Succeeded

So, we successfully failed.  :-)  Seriously, I think the bug here is that, 
just as you can't deselect the last drive using the mouse, you shouldn't 
be able to with the keyboard either.