Control the panel hiding speed in GNOME

Run gconf-editor and navigate to
/apps/panel/toplevels/top_panel_screen0/unhide_delay

"Specifies the number of milliseconds delay after the pointer enters the panel area before the panel is automatically re-shown. This key is only relevant if the auto_hide key is true. "

And of course..
/apps/panel/toplevels/top_panel_screen0/hide_delay

Specifies the number of milliseconds delay after the pointer leaves the panel area before the panel is automatically hidden. This key is only relevant if the auto_hide key is true.

And the same for
/apps/panel/toplevels/bottom_panel_screen0/unhide_delay
/apps/panel/toplevels/bottom_panel_screen0/hide_delay

Double click a line, change the value and OK it. Change takes effect immediately.