Hmm, so if you wipe to a boss fully and it leashes, it keeps the original loot (and GUID) but if you leash it and it vanishes entirely upon crossing the leash point, and then respawns anew a minute later, then the loot is rerandomized? Makes sense.

Doesn't Illidan despawn and reappear as a friendly NPC when you wipe to him?

Correct.

Why yes, yes he does.

Now, who didn't kill Illidan last night and wants to test this theory?

Well there's nothing to test on most servers. On Mal'Ganis someone with Warglaives will probably log on soon after a server reset and there's no way of really knowing for sure if they have or not. Certainly on most servers this won't work.

Also wouldn't it follow that the item has been "seen" on the server once it appears once in someone's instance, even if it's not obtained by a player, if this theory is true? So even if no one has any glaives, someone zoning into a BT instance in which a warglaive exists on Illidan's table might make the item linkable.

The only way to properly test this would be to know for a fact that no one on a given server has a piece of loot, and then to know for a fact that you are the first person to enter the zone in which that loot might spawn after a server reset. Right?

After you kill supremus you can /Target illidan. I don't know if this means he is actually there, I would just assume it does.

A blue stated that long ago during a discussion of I believe also loot generation. It was before the forum change so the post is certainly lost but might be available in some bluetracker. I don't have it at hand though.

That's right, though Slouken said that quite some time ago: WoW Forums -> Upcoming 2.4 Changes - Concise List

The "upper-middle" part is the mob-id. Taking Vaeys' example, Vexallus - NPCs - World of Warcraft is Vexallus. Enter 24744 in DEC, convert to Hex and you'll get 60A8. Compare with the combat log GUIDs:
WoWhead pulls these mob-id numbers out of the MPQ in one of the dbc files. So the lower 3 Bytes denote the variable ID, the following 2 (3?)Bytes denote the mob id and the uppermost part is probably some flags.

Loot is generated on spawn, its really an old argument and confirmed at least twice by blue. Also the community long ago before any confirmation found that there are specific mobs who wear the weapon they are going to drop IF they have that weapon in their loot table. I've seen and confirmed this myself. This would not be possible if loot is determined on kill. It really happens on spawn time.

So if we were to test this with 2.4 we need to track the bosses that are not there when the instance gets created but spawn later. We can assume this is true for Kil'Jaeden. It certainly isn't true for Kalecgos (unless Sartohvarr or whatever his name is, is spawned dynamically, as we don't loot Kalecgos after all). Brutallus is always present and Felmyst could be dynamically spawned as it is part of the event with Brutallus and Madrigosa. I have no idea about M'uru and Twin Eredars, save that M'uru has 2 phases and one probably loots Entropius.

Roana posted that at the very beginning of page 2 of this very same thread

Yes, that's how I understand the process. Since for a normal live realm you cannot guarantee you are the first to spawn a raid instance, it's probably easiest if one concentrates on observing later bosses which spawn in on player-interaction. So they are not present when the instance spawns and possibly they respawn on wipe.

It's easier to track server wide raid progression and with that the possibility to engage/activate a boss than trying to ensure no one else runs into the instance and spawns it.

If you furthermore are the first guild to reach a boss you can check to see if between each pull/attempt, the available loot for linking increases or not. If it does, you can for the first few pulls predict what he would drop. (Unless the entire loot table becomes active for linking once the boss spawns in for the first time)

I think 2.4 is a unique opportunity to gain a bit of insight into this mechanism.

I know this was the case with Everquest, but I've never seen anything similar in WoW. Do you have any examples?

Ouch of curiosity why can I link items like [Item not found!] or [Teebu's Blazing Longsword] without crashing? You'd think these items don't exist on the server just yet no? Or they're rare enough that a mob/GM wouldn't spawn with them.