This bug is ⌘W and ⌘⌥W close Inspectors too. It was filed on 2006-04-14 at 23:45 PDT.
When the user presses ⌘W, IB will close a floating window if it has focus. When the user presses ⌘⌥W to close all nib windows, IB also closes all of the floating windows.
Steps to Reproduce:
1. Press ⌘W or ⌘⌥W.
The frontmost non-floating window closes, or all non-floating windows and no floating windows close.
The frontmost window (floating or not) closes, or all windows (including floating windows) close.
Floating windows in IB are the Palette, the Alignment palette, and the Inspector.