Okay, this will ignite all kinds of "your crazy" flaming threads, and the like. Cities will burn, but it'll be okay :)

 Anyways, I'm going to break away from my former prediction of 10.5m for Halo 3, and wind up increasing it to 14.5m-16.5m.

 There are a few reasons for this, and just go with me, to explain why Halo 3 has this sort of capacity.

Reason #1. Uber-sellers must debut early (~3yrs) in the consoles lifespan.

This isn't always true for every game (GTA:SA), but has been true for the vast majority of games. FFVII, many GTA games, SM64, and such all debuted early in their respective consoles lifespan. When we talk Halo 2 numbers (which sold around 8.5m copies or more), we're talking a game that was incredibly late in the Xbox's lifespan. The console was DOA within 1 year of Halo's launch due to the X360. Despite this, it has managed good legs, selling about 4m units after it's o/w. Not only this, the important key is that the Xbox only managed to sell another 5 million hardware units - not enough to bring new blood into the Microsoft machine to sell more units.

On the other hand, Halo 3 has debuted rather early in the X360's lifespan, and it's an absolute lock that the X360 will surpass those numbers (6m+ consoles for the rest of it's lifespan). Because of this, the more hardware will simply allow for more software to be moved.

Reason #2 - No Comparible Game will exist to rival Halo 3.

Okay, this is very subjective, but nevertheless: I feel that it'll take quite a bit of time for a new IP, or franchise game to come out, and rival Halo 3 in terms of gameplay, and such. I'm just merely talking from an Xbox guys POV. In the case of Mario 64, and other Mario's, no game really compared to it for years. Likewise, GTA and Grant Turismo can say the same: they were the name brand racers and sandbox games for years. This is a key to garnering more sales. If Halo 3 can still be the name-brand FPS by this time next year, it's feasable it could move 500k or even 1m units during next Christmas. Remember, Halo 2 still moves 6k units/wk. Not much, but for 3 years on a totally dead console, that's very good (see any GC games topping the VGC 200 that are from 2004?).

Reason #3 - No proper sequel for quite some time.

As far as we can tell, no sequal is in development (H4) that would curb purchasing the game a few years down the road. Although we sometimes the prequel to a game get a boost from the new franchise releasing, it's usually short lived, and dies real quickly afterwards. If Halo 3 ends up being the final one, what prevents it from selling 15m units or more if the X360 still exists in 2010 as Microsoft's primary console, and has sold 45m hardware units? To some, that might be a high number, but still, if the X360 sold 45m units, I feel Halo3, if still the primary MS-IP could wind up with a 30-35% attach ratio.

Those are just some quick ideas I had. At either rate, I do feel that since the X360 has such a small, but growing fanbase and will easily surpass the Xbox's lifetime sales, even if not by much, and the fact the X360 has atleast 3 years left of sales before a sucessor releases, it'll do well.

Now, having said this, these 3 rules also apply to a few other major franchise games on other consoles such as Super Mario Galaxy, which the formula can really be seen in. Also, when I refer to no comparable games, I know we have KZ2 fanatics, and PC FPS junkies, but I am reffering to the X360 only. I am sure other viable FPS games might try to rival H3 on the other systems, but for the X360, it's doubtful that any will rival H3 for years.

I suppose it really depends on how much people really like the game. I heard alot of complaints about Halo 2 but don't know how well that sold.

In order for it to sell those kind of numbers I'd imagine the Xbox360 would have to sell in the region of 40-50million consoles

It won't sell 15 million for a variety of reasons. Mostly because the 360 user base won't significantly surpass the Xbox's. The 360 will most likely end up with around 32-35 million after 5 years and that's only around 10 mill more than the Xbox. Because of the extra consoles the 360 will sell, i've no doubt the game will end up around 10-11.5 million. If no comparable games for halo 3 will exist, then no comparable games for the first 2 existed and they still didn't do 10 million.

Look at the top 3 games on the N64 which will most likely be identical to the amount of 360's by the end of the generation.

Super Mario 64 Nintendo 11.89
Mario Kart 64 Nintendo 9.87
GoldenEye 007 Nintendo 8.09

That is the upper echelon of what a 35 million userbase game can sell. Reaching anywhere near Super Mario 64 will be some feat and that is the goal they should be shooting for. And when you think to yourself "Mario 64 didn't have the uber hype halo 3 does" then remember that the game was out on DAY ONE, IE 2 years earlier than halo 3 on it's console. That cancels out any extra hype halo 3 has.  If Halo 3 is a 10 on the hype scale, Mario 64 was a 9.

By the way...there isn't even a game on Ps2 aka 110 million consoles that did over 15 million according to this site. End of story. I know it's exciting and halo hype time right now, but you know when you really think about it, it won't come near 15 million. You're too smart to think it will with any confidence.

There's only 2 real CONSOLE games (according to THIS site) that have made over 15 million without being packins: Super mario bros 3 and Super Mario World. The rest are handhelds and duckhunt/mario packin types.  By the way, looking at those numbers there doesn't appear to ever have been a game that wasn't packed in with a console, that has ever done more than 33% of it's userbase.  Halo 3 will never do more than 12 million unless the 360 magically starts to sell better or lasts 6 years.

Your original estimation was right on the money in my opinion.  You should stick with it.   

You are crazy person,
It's got about as much chance as Wii hitting 200 million.

The base was bigger for Halo 2....

I wouldn't put it higher than 10mil personally

 I reckon Wii Play will be the biggest attach software ever after this generation.

8-10 M for me
but it is still big