the 30" apple display is discontinued and many years old, has a lower pixel density and is a different disign to your iMac... and i do not believe it is LED backlit.
the u2711 seems alot better suited it is the same size and ratio (16:9, apple 30" is 16:10) and pixel density as your iMac! You will still need the expensive dual link DVI cable to drive it.
Why do't you get the 27" Apple LED Cinema display, the perfect partner to your iMac. (Same design, screen, blacklight technology, size, ratio, pixel density and it is driven stright from the iMac mini display port etc)