I think a big chunk of sales is lost by the 'annual pass'
idk how many committed to the annual pass, but all those users are lost sales because they get the game for free.
I think I read something about 1million+

From those 18.5 million copies, one would have to assume that the largest slice must be from both Diablo II and LOD sales, since the original Diablo is more of a Cult classic than a full fledged multi million seller. 

I'm expecting a huge impact by Diablo III given the level of expectation that it has created over the past 11 years. Whether the impact will be positive or negative we still have to see, but the lack of PVP at the release date is a bit of a personal let down. 

Anyway, my CE is already pre-ordered so I'm just waiting for May 15 . 

5mil+ year one seems unlikely. As previously stated PC games like this have stupidely long legs. Unless blizzard changes their release cycles, this game will likely sell well for at least 5 years. I would guess 3-4mil first year, 10mil lifetime.

What so you didn't like the rampant cheating that went on during Diablo 1 on PC?  I remember all the fools that went around with regular God Mode.  They should have used Fake God Mode 2 which actually shows damage being taken off but when you get to 1 health (it fills to the top again like you just took a full health potion).  I could stand there for 10-15 mins with people hacking at me thinking they could kill me....  Yea I cheated for PVP...  but only after 80+% of the Diablo 1 user base started cheating.  Least I used the funnier cheats and gave people hope of one day killing me.

That is why they started trying to store your characters on bnet instead of your computer (so that it would be harder to cheat).  Reason why Diablo 2 open had more cheaters than if you just had your character on bnet (closed).

We don't really know how many of those users will be "lost sales" because we'll never know how many of those people would have bought Diablo III anyway without the promotion but you can be sure of one thing: Blizzard wouldn't be doing this promotion if they didn't think it would benefit them in the end.

My guess is that they want to "hook" WoW players into Diablo's world, turning them into new Diablo fans thus creating a bigger pool of potential buyers for the inevitable expansion down the line.

Plus, they just got money for an entire year of WoW from the player...  They aren't "losing" money.

Meh, Diablo 1 was easy enough with the Rogue class, using cheats on it was overkill. I was never much into PVP back on the Diablo days, I only discovered it's joys on DII, which while they were cheat free, it did a couple of extremely nagging exploits (SummonNecro lag, Bone Prison Town Portals, TP Killing, etc.) that were hair-tearing sometimes.

They better bring it to PS3 as the rumor goes and they will get more sales.

How about teleporting (using the chest armor piece that allows teleporting) hammerdins or the zeal pallys that somehow people could create way better ones than I ever could. My main was basically the noobish class in the book (trap assassin) but you should have seen my exploding traps on Baal runs (and my main lightning trap did pretty decent damage too).  Corpse explosion makes Baal runs last about 40 secs.

Diablo 1 it was just fun killing people but the main trick was not to kill them yourself.  The main trick was to damage them enough to the point of almost dying and then have a mob kill them.  Guess what?  You only dropped gold and an ear if you died from a player but if you died from a mob in Diablo 1 you dropped everything you have equiped.   LOL  they would be so pissed if you took their shit.  That is what made Diablo 1 PVP hilarious as hell.

Diablo 2 you didn't drop gear anymore when being killed by monsters.  So the only thing you could steal would be gold and an ear (I believe they still had those didn't they).