After docking the Web Inspector to the document that is being inspected, the window resize widget disappears. It is no longer possible to resize the window until you undock the Inspector.
The status bar needs to be hidden for this to happen and it only seems to happen for me on windows xp, and I can still resize the window despite there being no resize widget. Confirming regardless as the resizer still needs to be drawn.
<rdar://problem/5712789>
This seems to be fixed in Safari 4.