Designer's Island was, in fact, an island during the beta. It was beyond the water though so one had to...erm...do some exploiting' to get there.

After a few people were caught "harvesting" the items there, they all recieved 72 hour suspensions and the island was moved to an instance (thus, making it impossible to visit - anymore, they fixed one of the exploits).

What was there?

->Every type of ore, all lined up.
->Three, giant, level one squirrels - each which dropped some damn nice items (one dropped a horse and some other stuff, another dropped some test items [Bind on Pickup Test and Bind on Use Test], and the last one just dropped some shit).
->Every type of trainer, friendly to both Horde and Alliance.
->Every type of mount for sale, again the NPC's were friendly to both Horde and Alliance.
->There was, of course, a forge.
->Every type of herb, all lined up like the ore.
->A human child who ran around and gave a quest...something about Mr Wilson heh.
->A working cannon.
->A huge ship (of the boat variety) flying in circles in the air.
->A cart of explosives which could be blown up.
->A high-level (don't remember exactly what level) abomination.
->Rows and rows of NPC's standing in rows (if I remember correctly, hostile mages?).
->A *HUGE* sea-shell shaped building, presumably for the Naga - completely untextured.

The place itself was just a huge field (Westfall-like textured ground) with gently rolling hills. Then there was this one little valley with a farm-like field where most of the cool stuff was.

The other day on Worlddefense it showed up as being "attacked".  Guess the GMs were testing something.

Likely an instance now though, impossible for anyone to get into normally.  Though it wont stop me from trying to figure out how =)

Designer Island, not to be confused with programmer or GM island, is a completely inaccessible test area for WOW designers to develop and test new content.