Here’s an (non exhaustive) list of mac menu compatible themes for gtk :

• imetal
• snow leopard
• mac4lin
• kougyoku black
• and more kougyoku (make a search on http://gnome-look.org )

You can also hack your theme, like I did. I edited the gtkrc file of the theme I use. (the gtkrc file of your theme is located under ~/.themes/[YOUR_THEME_NAME]/gtk-2.0/gtkrc)

here’s the code I added in my gtkrc file :
…

# menu bar style
style "theme-menubar"
{
  xthickness            	= 0
  ythickness            	= 0
  #bg[NORMAL] = “#000000″
  bg_pixmap[NORMAL]		= “panel-bg.png”
  fg[NORMAL] 			= “#ffffff”
  fg[PRELIGHT] 		= “#ffffff”
  fg[ACTIVE] 			= “#555555″
  fg[SELECTED] 		= “#555555″
	engine “pixmap”
	{
		image
		{
			function	= BOX
			state = NORMAL
			file		= “panel-bg.png”
			border	= { 0, 0, 0, 0 }
			stretch	= TRUE
    		}
    }
}

class “GtkMenuBar” style “theme-menubar”

# panel style
style “panelbg”
{
  xthickness            	= 0
  ythickness            	= 0
  bg_pixmap[NORMAL]		= “panel-bg.png”
  fg[NORMAL] 			= “#ffffff”
  fg[PRELIGHT] 		= “#ffffff”
  fg[ACTIVE] 			= “#555555″
  fg[SELECTED] 		= “#555555″

}
#class “*Panel*” style “panelbg”
widget “*PanelApplet*” style “panelbg”
class “*notif*” style “panelbg”
class “*Notif*” style “panelbg”
class “*Tray*” style “panelbg”
class “*tray*” style “panelbg”
class “*Netstatus*” style “panelbg”
class “*netstatus*” style “panelbg”
class “*Deskbar*” style “panelbg”
class “*deskbar*” style “panelbg”

I saved the panel-bg.png image in the same directory as the gtkrc file, and also used this png as panel background image (right click on the panel, choose properties, then background tab where you specify the background image location).
Note: you may have to play with the height of the png file and the height of the panel (using panel properties dialog)